diff --git a/src/knowledge_base.cpp b/src/knowledge_base.cpp
index 51bfde1..a0209dd 100644
--- a/src/knowledge_base.cpp
+++ b/src/knowledge_base.cpp
@@ -1,13 +1,16 @@
 #include "knowledge_base.h"
 #include "models/action.h"
 #include "models/entity.h"
+#include "models/error.h"
 #include "models/fact.h"
 #include "models/rule.h"
 #include "models/suggest_action.h"
 #include "models/verb.h"
 
+#include <cstring>
 #include <filesystem>
 #include <iostream>
+#include <string>
 
 obelisk::KnowledgeBase::KnowledgeBase(const char* filename, int flags)
 {
@@ -23,6 +26,8 @@ obelisk::KnowledgeBase::KnowledgeBase(const char* filename, int flags)
         logSqliteError(result);
     }
 
+    enableForeignKeys();
+
     if (!dbExists)
     {
         createTable(obelisk::Action::createTable);
@@ -42,16 +47,25 @@ obelisk::KnowledgeBase::~KnowledgeBase()
     }
 }
 
-void obelisk::KnowledgeBase::createTable(std::function<const char*()> function)
+/**
+ * @brief Enable foreign key functionality in the open database.
+ *
+ * This must always be done when the connection is opened or it will not
+ * enforce the foreign key constraints.
+ */
+void obelisk::KnowledgeBase::enableForeignKeys()
 {
-    char* tmp;
-    auto result = sqlite3_exec(dbConnection_, function(), NULL, NULL, &tmp);
+    char* errmsg;
+    int result = sqlite3_exec(dbConnection_,
+        "PRAGMA foreign_keys = ON;",
+        NULL,
+        NULL,
+        &errmsg);
     if (result != SQLITE_OK)
     {
         logSqliteError(result);
-        if (tmp)
+        if (errmsg)
         {
-            std::string errmsg(tmp);
             throw obelisk::KnowledgeBaseException(errmsg);
         }
         else
@@ -61,53 +75,103 @@ void obelisk::KnowledgeBase::createTable(std::function<const char*()> function)
     }
 }
 
-// TODO: change these to throw errors instead of return int
-int obelisk::KnowledgeBase::addEntities(std::vector<obelisk::Entity>& entities)
+void obelisk::KnowledgeBase::createTable(std::function<const char*()> function)
+{
+    char* errmsg;
+    int result = sqlite3_exec(dbConnection_, function(), NULL, NULL, &errmsg);
+    if (result != SQLITE_OK)
+    {
+        logSqliteError(result);
+        if (errmsg)
+        {
+            throw obelisk::KnowledgeBaseException(errmsg);
+        }
+        else
+        {
+            throw obelisk::KnowledgeBaseException();
+        }
+    }
+}
+
+void obelisk::KnowledgeBase::addEntities(std::vector<obelisk::Entity>& entities)
 {
     for (auto& entity : entities)
     {
-        entity.insertEntity(dbConnection_);
+        try
+        {
+            entity.insertEntity(dbConnection_);
+        }
+        catch (obelisk::DatabaseException::ConstraintException& exception)
+        {
+            // ignore unique constraint error
+            if (std::strcmp(exception.what(),
+                    "UNIQUE constraint failed: entity.name")
+                != 0)
+            {
+                throw;
+            }
+        }
     }
-    return 0;
 }
 
-int obelisk::KnowledgeBase::addVerbs(std::vector<obelisk::Verb>& verbs)
+void obelisk::KnowledgeBase::addVerbs(std::vector<obelisk::Verb>& verbs)
 {
     for (auto& verb : verbs)
     {
-        verb.insertVerb(dbConnection_);
+        try
+        {
+            verb.insertVerb(dbConnection_);
+        }
+        catch (obelisk::DatabaseException::ConstraintException& exception)
+        {
+            // ignore unique constraint error
+            if (std::strcmp(exception.what(),
+                    "UNIQUE constraint failed: verb.name")
+                != 0)
+            {
+                throw;
+            }
+        }
     }
-    return 0;
 }
 
-int obelisk::KnowledgeBase::addFacts(std::vector<obelisk::Fact>& facts)
+void obelisk::KnowledgeBase::addFacts(std::vector<obelisk::Fact>& facts)
 {
     for (auto& fact : facts)
     {
-        fact.insertFact(dbConnection_);
+        try
+        {
+            fact.insertFact(dbConnection_);
+        }
+        catch (obelisk::DatabaseException::ConstraintException& exception)
+        {
+            // ignore unique constraint error
+            if (std::strcmp(exception.what(),
+                    "UNIQUE constraint failed: fact.left_entity, fact.right_entity, fact.verb")
+                != 0)
+            {
+                throw;
+            }
+        }
     }
-    return 0;
 }
 
-int obelisk::KnowledgeBase::getEntity(obelisk::Entity& entity)
+void obelisk::KnowledgeBase::getEntity(obelisk::Entity& entity)
 {
     entity.selectEntity(dbConnection_);
-    return 0;
 }
 
-int obelisk::KnowledgeBase::getVerb(obelisk::Verb& verb)
+void obelisk::KnowledgeBase::getVerb(obelisk::Verb& verb)
 {
     verb.selectVerb(dbConnection_);
-    return 0;
 }
 
-int obelisk::KnowledgeBase::getFact(obelisk::Fact& fact)
+void obelisk::KnowledgeBase::getFact(obelisk::Fact& fact)
 {
     fact.selectFact(dbConnection_);
-    return 0;
 }
 
-// TODO: log files?
+// TODO: log files? or just throw an error?
 void obelisk::KnowledgeBase::logSqliteError(int result)
 {
     std::cout << sqlite3_errstr(result) << std::endl;
diff --git a/src/knowledge_base.h b/src/knowledge_base.h
index e690fe8..7458f65 100644
--- a/src/knowledge_base.h
+++ b/src/knowledge_base.h
@@ -24,6 +24,7 @@ namespace obelisk
             int flags_;
             void logSqliteError(int result);
 
+            void enableForeignKeys();
             void createTable(std::function<const char*()> function);
 
         public:
@@ -37,13 +38,13 @@ namespace obelisk
 
             ~KnowledgeBase();
 
-            int addEntities(std::vector<obelisk::Entity>& entities);
-            int addVerbs(std::vector<obelisk::Verb>& verbs);
-            int addFacts(std::vector<obelisk::Fact>& facts);
+            void addEntities(std::vector<obelisk::Entity>& entities);
+            void addVerbs(std::vector<obelisk::Verb>& verbs);
+            void addFacts(std::vector<obelisk::Fact>& facts);
 
-            int getEntity(obelisk::Entity& entity);
-            int getVerb(obelisk::Verb& verb);
-            int getFact(obelisk::Fact& fact);
+            void getEntity(obelisk::Entity& entity);
+            void getVerb(obelisk::Verb& verb);
+            void getFact(obelisk::Fact& fact);
 
             void getDouble(double& result, float var1, float var2);
             void getFloat(float& result1, float& result2, double var);
diff --git a/src/models/entity.cpp b/src/models/entity.cpp
index 7089841..d7e81a8 100644
--- a/src/models/entity.cpp
+++ b/src/models/entity.cpp
@@ -1,4 +1,5 @@
 #include "models/entity.h"
+#include "models/error.h"
 
 const char* obelisk::Entity::createTable()
 {
@@ -11,118 +12,139 @@ const char* obelisk::Entity::createTable()
     )";
 }
 
-int obelisk::Entity::selectEntity(sqlite3* dbConnection)
+void obelisk::Entity::selectEntity(sqlite3* dbConnection)
 {
-    // TODO: check if database is open
+    if (dbConnection == nullptr)
+    {
+        throw obelisk::DatabaseException("database isn't open");
+    }
+
     sqlite3_stmt* ppStmt = nullptr;
-    const char* pzTail   = nullptr;
 
     auto result = sqlite3_prepare_v2(dbConnection,
-        "SELECT id, name FROM entity WHERE name=?;",
+        "SELECT id, name FROM entity WHERE name=?",
         -1,
         &ppStmt,
-        &pzTail);
+        nullptr);
+
     if (result != SQLITE_OK)
     {
-        // TODO: something went wrong throw an error
+        throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
     }
 
-    if (pzTail != nullptr)
+    result = sqlite3_bind_text(ppStmt, 1, getName().c_str(), -1, SQLITE_STATIC);
+    switch (result)
     {
-        // TODO: Something was not used... throw an error
-    }
-
-    result
-        = sqlite3_bind_text(ppStmt, 1, getName().c_str(), -1, SQLITE_TRANSIENT);
-    if (result != SQLITE_OK)
-    {
-        // TODO: Something is wrong... throw an error
+        case SQLITE_OK :
+            break;
+        case SQLITE_TOOBIG :
+            throw obelisk::DatabaseException::SizeException();
+            break;
+        case SQLITE_RANGE :
+            throw obelisk::DatabaseException::RangeException();
+            break;
+        case SQLITE_NOMEM :
+            throw obelisk::DatabaseException::MemoryException();
+            break;
+        default :
+            throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+            break;
     }
 
     result = sqlite3_step(ppStmt);
-    if (result != SQLITE_DONE)
+    switch (result)
     {
-        // TODO: Something is wrong... throw an error
+        case SQLITE_DONE :
+            // no rows in the database
+            break;
+        case SQLITE_ROW :
+            setId(sqlite3_column_int(ppStmt, 0));
+            setName((char*) sqlite3_column_text(ppStmt, 1));
+            break;
+        case SQLITE_BUSY :
+            throw obelisk::DatabaseException::BusyException();
+            break;
+        case SQLITE_MISUSE :
+            throw obelisk::DatabaseException::MisuseException();
+            break;
+        default :
+            throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+            break;
     }
 
-    if (result == SQLITE_ROW)
-    {
-        setId(sqlite3_column_int(ppStmt, 0));
-        setName((char*) sqlite3_column_text(ppStmt, 1));
+    result = sqlite3_finalize(ppStmt);
 
-        result = sqlite3_finalize(ppStmt);
-        if (result != SQLITE_OK)
-        {
-            // TODO: Something is wrong... throw an error
-        }
-        return 0;
-    }
-    else
+    if (result != SQLITE_OK)
     {
-        result = sqlite3_finalize(ppStmt);
-        if (result != SQLITE_OK)
-        {
-            // TODO: Something is wrong... throw an error
-        }
-        return 0;
+        throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
     }
 }
 
-int obelisk::Entity::insertEntity(sqlite3* dbConnection)
+void obelisk::Entity::insertEntity(sqlite3* dbConnection)
 {
-    // TODO: check if database is open
-
-    /*selectEntity(dbConnection);
-    if (getId() != 0)
+    if (dbConnection == nullptr)
     {
-        // TODO: already exists in database, throw an error? Or skip past it?
-        return -1;
-    }*/
-
-    sqlite3_stmt* ppStmt = nullptr;
-    const char* pzTail   = nullptr;
-
-    auto result = sqlite3_prepare_v2(dbConnection,
-        "INSERT INTO entity (name) VALUES (?);",
-        -1,
-        &ppStmt,
-        &pzTail);
-    if (result != SQLITE_OK)
-    {
-        // TODO: something went wrong throw an error
+        throw obelisk::DatabaseException("database isn't open");
     }
 
-    if (pzTail != nullptr)
+    sqlite3_stmt* ppStmt = nullptr;
+
+    auto result = sqlite3_prepare_v2(dbConnection,
+        "INSERT INTO entity (name) VALUES (?)",
+        -1,
+        &ppStmt,
+        nullptr);
+    if (result != SQLITE_OK)
     {
-        // TODO: Something was not used... throw an error
+        throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
     }
 
     result
         = sqlite3_bind_text(ppStmt, 1, getName().c_str(), -1, SQLITE_TRANSIENT);
-    if (result != SQLITE_OK)
+    switch (result)
     {
-        // TODO: Something is wrong... throw an error
+        case SQLITE_OK :
+            break;
+        case SQLITE_TOOBIG :
+            throw obelisk::DatabaseException::SizeException();
+            break;
+        case SQLITE_RANGE :
+            throw obelisk::DatabaseException::RangeException();
+            break;
+        case SQLITE_NOMEM :
+            throw obelisk::DatabaseException::MemoryException();
+            break;
+        default :
+            throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+            break;
     }
 
     result = sqlite3_step(ppStmt);
-    if (result != SQLITE_DONE)
+    switch (result)
     {
-        // TODO: Something is wrong... throw an error
+        case SQLITE_DONE :
+            setId((int) sqlite3_last_insert_rowid(dbConnection));
+            sqlite3_set_last_insert_rowid(dbConnection, 0);
+            break;
+        case SQLITE_CONSTRAINT :
+            throw obelisk::DatabaseException::ConstraintException(
+                sqlite3_errmsg(dbConnection));
+        case SQLITE_BUSY :
+            throw obelisk::DatabaseException::BusyException();
+            break;
+        case SQLITE_MISUSE :
+            throw obelisk::DatabaseException::MisuseException();
+            break;
+        default :
+            throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+            break;
     }
-    else
-    {
-        setId((int) sqlite3_last_insert_rowid(dbConnection));
-    }
-
-    sqlite3_set_last_insert_rowid(dbConnection, 0);
 
     result = sqlite3_finalize(ppStmt);
     if (result != SQLITE_OK)
     {
-        // TODO: Something is wrong... throw an error
+        throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
     }
-
-    return 0;
 }
 
 int& obelisk::Entity::getId()
diff --git a/src/models/entity.h b/src/models/entity.h
index f799f31..7e4d0e0 100644
--- a/src/models/entity.h
+++ b/src/models/entity.h
@@ -46,8 +46,8 @@ namespace obelisk
             std::string& getName();
             void setName(std::string name);
 
-            int selectEntity(sqlite3* dbConnection);
-            int insertEntity(sqlite3* dbConnection);
+            void selectEntity(sqlite3* dbConnection);
+            void insertEntity(sqlite3* dbConnection);
     };
 } // namespace obelisk
 
diff --git a/src/models/error.h b/src/models/error.h
new file mode 100644
index 0000000..e8ace1b
--- /dev/null
+++ b/src/models/error.h
@@ -0,0 +1,146 @@
+#ifndef OBELISK_MODELS_ERROR_H
+#define OBELISK_MODELS_ERROR_H
+
+#include <exception>
+#include <string>
+
+namespace obelisk
+{
+    class DatabaseException : public std::exception
+    {
+        private:
+            const std::string errorMessage_;
+
+        public:
+            DatabaseException() :
+                errorMessage_("an unknown error ocurred")
+            {
+            }
+
+            DatabaseException(const int errorCode) :
+                errorMessage_(
+                    "database error " + std::to_string(errorCode) + " ocurred")
+            {
+            }
+
+            DatabaseException(const std::string& errorMessage) :
+                errorMessage_(errorMessage)
+            {
+            }
+
+            const char* what() const noexcept
+            {
+                return errorMessage_.c_str();
+            }
+
+            class SizeException : public std::exception
+            {
+                private:
+                    const std::string errorMessage_;
+
+                public:
+                    SizeException() :
+                        errorMessage_("size of string or blob exceeds limits")
+                    {
+                    }
+
+                    const char* what() const noexcept
+                    {
+                        return errorMessage_.c_str();
+                    }
+            };
+
+            class RangeException : public std::exception
+            {
+                private:
+                    const std::string errorMessage_;
+
+                public:
+                    RangeException() :
+                        errorMessage_("parameter index is out of range")
+                    {
+                    }
+
+                    const char* what() const noexcept
+                    {
+                        return errorMessage_.c_str();
+                    }
+            };
+
+            class MemoryException : public std::exception
+            {
+                private:
+                    const std::string errorMessage_;
+
+                public:
+                    MemoryException() :
+                        errorMessage_("not enough memory for operation")
+                    {
+                    }
+
+                    const char* what() const noexcept
+                    {
+                        return errorMessage_.c_str();
+                    }
+            };
+
+            class BusyException : public std::exception
+            {
+                private:
+                    const std::string errorMessage_;
+
+                public:
+                    BusyException() :
+                        errorMessage_(
+                            "database was busy and operation not performed")
+                    {
+                    }
+
+                    const char* what() const noexcept
+                    {
+                        return errorMessage_.c_str();
+                    }
+            };
+
+            class MisuseException : public std::exception
+            {
+                private:
+                    const std::string errorMessage_;
+
+                public:
+                    MisuseException() :
+                        errorMessage_("misuse of the database routine")
+                    {
+                    }
+
+                    const char* what() const noexcept
+                    {
+                        return errorMessage_.c_str();
+                    }
+            };
+
+            class ConstraintException : public std::exception
+            {
+                private:
+                    const std::string errorMessage_;
+
+                public:
+                    ConstraintException() :
+                        errorMessage_("a constraint exception occurred")
+                    {
+                    }
+
+                    ConstraintException(const std::string& errorMessage) :
+                        errorMessage_(errorMessage)
+                    {
+                    }
+
+                    const char* what() const noexcept
+                    {
+                        return errorMessage_.c_str();
+                    }
+            };
+    };
+} // namespace obelisk
+
+#endif
diff --git a/src/models/fact.cpp b/src/models/fact.cpp
index 51c6c40..467373d 100644
--- a/src/models/fact.cpp
+++ b/src/models/fact.cpp
@@ -1,3 +1,4 @@
+#include "models/error.h"
 #include "models/fact.h"
 
 const char* obelisk::Fact::createTable()
@@ -9,6 +10,7 @@ const char* obelisk::Fact::createTable()
             "right_entity" INTEGER NOT NULL,
             "verb"         INTEGER NOT NULL,
             PRIMARY KEY("id" AUTOINCREMENT),
+            UNIQUE("left_entity", "right_entity", "verb")
             FOREIGN KEY("verb") REFERENCES "verb"("id") ON DELETE RESTRICT,
             FOREIGN KEY("right_entity") REFERENCES "entity"("id") ON DELETE RESTRICT,
             FOREIGN KEY("left_entity") REFERENCES "entity"("id") ON DELETE RESTRICT
@@ -16,146 +18,214 @@ const char* obelisk::Fact::createTable()
     )";
 }
 
-int obelisk::Fact::selectFact(sqlite3* dbConnection)
+void obelisk::Fact::selectFact(sqlite3* dbConnection)
 {
-    // TODO: check if database is open
-
-    sqlite3_stmt* ppStmt = nullptr;
-    const char* pzTail   = nullptr;
-
-    auto result = sqlite3_prepare_v2(dbConnection,
-        "SELECT id, left_entity, right_entity, verb FROM fact WHERE (left_entity=? AND right_entity=? AND verb=?);",
-        -1,
-        &ppStmt,
-        &pzTail);
-    if (result != SQLITE_OK)
+    if (dbConnection == nullptr)
     {
-        // TODO: something went wrong throw an error
+        throw obelisk::DatabaseException("database isn't open");
     }
 
-    if (pzTail != nullptr)
+    sqlite3_stmt* ppStmt = nullptr;
+
+    auto result = sqlite3_prepare_v2(dbConnection,
+        "SELECT id, left_entity, right_entity, verb FROM fact WHERE (left_entity=? AND right_entity=? AND verb=?)",
+        -1,
+        &ppStmt,
+        nullptr);
+    if (result != SQLITE_OK)
     {
-        // TODO: Something was not used... throw an error
+        throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
     }
 
     result = sqlite3_bind_int(ppStmt, 1, getLeftEntity().getId());
-    if (result != SQLITE_OK)
+    switch (result)
     {
-        // TODO: Something is wrong... throw an error
+        case SQLITE_OK :
+            break;
+        case SQLITE_TOOBIG :
+            throw obelisk::DatabaseException::SizeException();
+            break;
+        case SQLITE_RANGE :
+            throw obelisk::DatabaseException::RangeException();
+            break;
+        case SQLITE_NOMEM :
+            throw obelisk::DatabaseException::MemoryException();
+            break;
+        default :
+            throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+            break;
     }
 
     result = sqlite3_bind_int(ppStmt, 2, getRightEntity().getId());
-    if (result != SQLITE_OK)
+    switch (result)
     {
-        // TODO: Something is wrong... throw an error
+        case SQLITE_OK :
+            break;
+        case SQLITE_TOOBIG :
+            throw obelisk::DatabaseException::SizeException();
+            break;
+        case SQLITE_RANGE :
+            throw obelisk::DatabaseException::RangeException();
+            break;
+        case SQLITE_NOMEM :
+            throw obelisk::DatabaseException::MemoryException();
+            break;
+        default :
+            throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+            break;
     }
 
     result = sqlite3_bind_int(ppStmt, 3, getVerb().getId());
-    if (result != SQLITE_OK)
+    switch (result)
     {
-        // TODO: Something is wrong... throw an error
+        case SQLITE_OK :
+            break;
+        case SQLITE_TOOBIG :
+            throw obelisk::DatabaseException::SizeException();
+            break;
+        case SQLITE_RANGE :
+            throw obelisk::DatabaseException::RangeException();
+            break;
+        case SQLITE_NOMEM :
+            throw obelisk::DatabaseException::MemoryException();
+            break;
+        default :
+            throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+            break;
     }
 
     result = sqlite3_step(ppStmt);
-    if (result != SQLITE_DONE)
+    switch (result)
     {
-        // TODO: Something is wrong... throw an error
+        case SQLITE_DONE :
+            // no rows in the database
+            break;
+        case SQLITE_ROW :
+            setId(sqlite3_column_int(ppStmt, 0));
+            getLeftEntity().setId(sqlite3_column_int(ppStmt, 1));
+            getRightEntity().setId(sqlite3_column_int(ppStmt, 2));
+            getVerb().setId(sqlite3_column_int(ppStmt, 3));
+            break;
+        case SQLITE_BUSY :
+            throw obelisk::DatabaseException::BusyException();
+            break;
+        case SQLITE_MISUSE :
+            throw obelisk::DatabaseException::MisuseException();
+            break;
+        default :
+            throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+            break;
     }
 
-    if (result == SQLITE_ROW)
-    {
-        setId(sqlite3_column_int(ppStmt, 0));
-        getLeftEntity().setId(sqlite3_column_int(ppStmt, 1));
-        getRightEntity().setId(sqlite3_column_int(ppStmt, 2));
-        getVerb().setId(sqlite3_column_int(ppStmt, 3));
-
-        result = sqlite3_finalize(ppStmt);
-        if (result != SQLITE_OK)
-        {
-            // TODO: Something is wrong... throw an error
-        }
-        return 0;
-    }
-    else
-    {
-        result = sqlite3_finalize(ppStmt);
-        if (result != SQLITE_OK)
-        {
-            // TODO: Something is wrong... throw an error
-        }
-        return 0;
-    }
-}
-
-int obelisk::Fact::insertFact(sqlite3* dbConnection)
-{
-    // TODO: make sure database is open
-
-    // check if the fact id exists, based on the ids of the entities and verb
-    /*selectFact(dbConnection);
-    if (getId() != 0)
-    {
-        // TODO: Verb is already in database, throw an error? Or just skip it?
-        return -1;
-    }*/
-
-    // TODO: verify that verbId, leftEntityId, and rightEntityId are not 0
-
-    sqlite3_stmt* ppStmt = nullptr;
-    const char* pzTail   = nullptr;
-
-    auto result = sqlite3_prepare_v2(dbConnection,
-        "INSERT INTO fact (left_entity, right_entity, verb) VALUES (?, ?, ?);",
-        -1,
-        &ppStmt,
-        &pzTail);
-    if (result != SQLITE_OK)
-    {
-        // TODO: something went wrong throw an error
-    }
-
-    if (pzTail != nullptr)
-    {
-        // TODO: Something was not used... throw an error
-    }
-
-    result = sqlite3_bind_int(ppStmt, 1, getLeftEntity().getId());
-    if (result != SQLITE_OK)
-    {
-        // TODO: Something is wrong... throw an error
-    }
-
-    result = sqlite3_bind_int(ppStmt, 2, getRightEntity().getId());
-    if (result != SQLITE_OK)
-    {
-        // TODO: Something is wrong... throw an error
-    }
-
-    result = sqlite3_bind_int(ppStmt, 3, getVerb().getId());
-    if (result != SQLITE_OK)
-    {
-        // TODO: Something is wrong... throw an error
-    }
-
-    result = sqlite3_step(ppStmt);
-    if (result != SQLITE_DONE)
-    {
-        // TODO: Something is wrong... throw an error
-    }
-    else
-    {
-        setId((int) sqlite3_last_insert_rowid(dbConnection));
-    }
-
-    sqlite3_set_last_insert_rowid(dbConnection, 0);
-
     result = sqlite3_finalize(ppStmt);
     if (result != SQLITE_OK)
     {
-        // TODO: Something is wrong... throw an error
+        throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+    }
+}
+
+void obelisk::Fact::insertFact(sqlite3* dbConnection)
+{
+    if (dbConnection == nullptr)
+    {
+        throw obelisk::DatabaseException("database isn't open");
     }
 
-    return 0;
+    sqlite3_stmt* ppStmt = nullptr;
+
+    auto result = sqlite3_prepare_v2(dbConnection,
+        "INSERT INTO fact (left_entity, right_entity, verb) VALUES (?, ?, ?)",
+        -1,
+        &ppStmt,
+        nullptr);
+    if (result != SQLITE_OK)
+    {
+        throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+    }
+
+    result = sqlite3_bind_int(ppStmt, 1, getLeftEntity().getId());
+    switch (result)
+    {
+        case SQLITE_OK :
+            break;
+        case SQLITE_TOOBIG :
+            throw obelisk::DatabaseException::SizeException();
+            break;
+        case SQLITE_RANGE :
+            throw obelisk::DatabaseException::RangeException();
+            break;
+        case SQLITE_NOMEM :
+            throw obelisk::DatabaseException::MemoryException();
+            break;
+        default :
+            throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+            break;
+    }
+
+    result = sqlite3_bind_int(ppStmt, 2, getRightEntity().getId());
+    switch (result)
+    {
+        case SQLITE_OK :
+            break;
+        case SQLITE_TOOBIG :
+            throw obelisk::DatabaseException::SizeException();
+            break;
+        case SQLITE_RANGE :
+            throw obelisk::DatabaseException::RangeException();
+            break;
+        case SQLITE_NOMEM :
+            throw obelisk::DatabaseException::MemoryException();
+            break;
+        default :
+            throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+            break;
+    }
+
+    result = sqlite3_bind_int(ppStmt, 3, getVerb().getId());
+    switch (result)
+    {
+        case SQLITE_OK :
+            break;
+        case SQLITE_TOOBIG :
+            throw obelisk::DatabaseException::SizeException();
+            break;
+        case SQLITE_RANGE :
+            throw obelisk::DatabaseException::RangeException();
+            break;
+        case SQLITE_NOMEM :
+            throw obelisk::DatabaseException::MemoryException();
+            break;
+        default :
+            throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+            break;
+    }
+
+    result = sqlite3_step(ppStmt);
+    switch (result)
+    {
+        case SQLITE_DONE :
+            setId((int) sqlite3_last_insert_rowid(dbConnection));
+            sqlite3_set_last_insert_rowid(dbConnection, 0);
+            break;
+        case SQLITE_CONSTRAINT :
+            throw obelisk::DatabaseException::ConstraintException(
+                sqlite3_errmsg(dbConnection));
+        case SQLITE_BUSY :
+            throw obelisk::DatabaseException::BusyException();
+            break;
+        case SQLITE_MISUSE :
+            throw obelisk::DatabaseException::MisuseException();
+            break;
+        default :
+            throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+            break;
+    }
+
+    result = sqlite3_finalize(ppStmt);
+    if (result != SQLITE_OK)
+    {
+        throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+    }
 }
 
 int& obelisk::Fact::getId()
diff --git a/src/models/fact.h b/src/models/fact.h
index 332f089..0bdf0ef 100644
--- a/src/models/fact.h
+++ b/src/models/fact.h
@@ -69,8 +69,8 @@ namespace obelisk
             Verb& getVerb();
             void setVerb(obelisk::Verb verb);
 
-            int selectFact(sqlite3* dbConnection);
-            int insertFact(sqlite3* dbConnection);
+            void selectFact(sqlite3* dbConnection);
+            void insertFact(sqlite3* dbConnection);
     };
 } // namespace obelisk
 
diff --git a/src/models/rule.cpp b/src/models/rule.cpp
index e9bde88..7dfc4f5 100644
--- a/src/models/rule.cpp
+++ b/src/models/rule.cpp
@@ -8,6 +8,7 @@ const char* obelisk::Rule::createTable()
             "fact"   INTEGER NOT NULL,
             "reason" INTEGER NOT NULL CHECK("reason" != "fact"),
             PRIMARY KEY("id" AUTOINCREMENT),
+            UNIQUE("fact", "reason"),
             FOREIGN KEY("fact") REFERENCES "fact"("id") ON DELETE RESTRICT,
             FOREIGN KEY("reason") REFERENCES "fact"("id") ON DELETE RESTRICT
         );
diff --git a/src/models/suggest_action.cpp b/src/models/suggest_action.cpp
index 929aa5b..838a33b 100644
--- a/src/models/suggest_action.cpp
+++ b/src/models/suggest_action.cpp
@@ -9,6 +9,7 @@ const char* obelisk::SuggestAction::createTable()
             "true_action"  INTEGER NOT NULL,
             "false_action" INTEGER NOT NULL,
             PRIMARY KEY("id" AUTOINCREMENT),
+            UNIQUE("fact", "true_action", "false_action"),
             FOREIGN KEY("fact") REFERENCES "fact"("id") ON DELETE RESTRICT,
             FOREIGN KEY("true_action") REFERENCES "action"("id") ON DELETE RESTRICT,
             FOREIGN KEY("false_action") REFERENCES "action"("id") ON DELETE RESTRICT
diff --git a/src/models/verb.cpp b/src/models/verb.cpp
index a49b3bd..9649533 100644
--- a/src/models/verb.cpp
+++ b/src/models/verb.cpp
@@ -1,3 +1,4 @@
+#include "models/error.h"
 #include "models/verb.h"
 
 #include <iostream>
@@ -13,119 +14,137 @@ const char* obelisk::Verb::createTable()
     )";
 }
 
-int obelisk::Verb::selectVerb(sqlite3* dbConnection)
+void obelisk::Verb::selectVerb(sqlite3* dbConnection)
 {
-    // TODO: check if database is open
+    if (dbConnection == nullptr)
+    {
+        throw obelisk::DatabaseException("database isn't open");
+    }
 
     sqlite3_stmt* ppStmt = nullptr;
-    const char* pzTail   = nullptr;
 
     auto result = sqlite3_prepare_v2(dbConnection,
-        "SELECT id, name FROM verb WHERE name=?;",
+        "SELECT id, name FROM verb WHERE name=?",
         -1,
         &ppStmt,
-        &pzTail);
+        nullptr);
     if (result != SQLITE_OK)
     {
-        // TODO: something went wrong throw an error
+        throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
     }
 
-    if (pzTail != nullptr)
+    result = sqlite3_bind_text(ppStmt, 1, getName().c_str(), -1, SQLITE_STATIC);
+    switch (result)
     {
-        // TODO: Something was not used... throw an error
-    }
-
-    result
-        = sqlite3_bind_text(ppStmt, 1, getName().c_str(), -1, SQLITE_TRANSIENT);
-    if (result != SQLITE_OK)
-    {
-        // TODO: Something is wrong... throw an error
+        case SQLITE_OK :
+            break;
+        case SQLITE_TOOBIG :
+            throw obelisk::DatabaseException::SizeException();
+            break;
+        case SQLITE_RANGE :
+            throw obelisk::DatabaseException::RangeException();
+            break;
+        case SQLITE_NOMEM :
+            throw obelisk::DatabaseException::MemoryException();
+            break;
+        default :
+            throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+            break;
     }
 
     result = sqlite3_step(ppStmt);
-    if (result != SQLITE_DONE)
+    switch (result)
     {
-        // TODO: Something is wrong... throw an error
+        case SQLITE_DONE :
+            // no rows in the database
+            break;
+        case SQLITE_ROW :
+            setId(sqlite3_column_int(ppStmt, 0));
+            setName((char*) sqlite3_column_text(ppStmt, 1));
+            break;
+        case SQLITE_BUSY :
+            throw obelisk::DatabaseException::BusyException();
+            break;
+        case SQLITE_MISUSE :
+            throw obelisk::DatabaseException::MisuseException();
+            break;
+        default :
+            throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+            break;
     }
 
-    if (result == SQLITE_ROW)
-    {
-        setId(sqlite3_column_int(ppStmt, 0));
-        setName((char*) sqlite3_column_text(ppStmt, 1));
-
-        result = sqlite3_finalize(ppStmt);
-        if (result != SQLITE_OK)
-        {
-            // TODO: Something is wrong... throw an error
-        }
-        return 0;
-    }
-    else
-    {
-        result = sqlite3_finalize(ppStmt);
-        if (result != SQLITE_OK)
-        {
-            // TODO: Something is wrong... throw an error
-        }
-        return 0;
-    }
-}
-
-int obelisk::Verb::insertVerb(sqlite3* dbConnection)
-{
-    // TODO: make sure database is open
-
-    /*selectVerb(dbConnection);
-    if (getId() != 0)
-    {
-        // TODO: Verb is already in database, throw an error? Or just skip it?
-        return -1;
-    }*/
-
-    sqlite3_stmt* ppStmt = nullptr;
-    const char* pzTail   = nullptr;
-
-    auto result = sqlite3_prepare_v2(dbConnection,
-        "INSERT INTO verb (name) VALUES (?);",
-        -1,
-        &ppStmt,
-        &pzTail);
-    if (result != SQLITE_OK)
-    {
-        // TODO: something went wrong throw an error
-    }
-
-    if (pzTail != nullptr)
-    {
-        // TODO: Something was not used... throw an error
-    }
-
-    result
-        = sqlite3_bind_text(ppStmt, 1, getName().c_str(), -1, SQLITE_TRANSIENT);
-    if (result != SQLITE_OK)
-    {
-        // TODO: Something is wrong... throw an error
-    }
-
-    result = sqlite3_step(ppStmt);
-    if (result != SQLITE_DONE)
-    {
-        // TODO: Something is wrong... throw an error
-    }
-    else
-    {
-        setId((int) sqlite3_last_insert_rowid(dbConnection));
-    }
-
-    sqlite3_set_last_insert_rowid(dbConnection, 0);
-
     result = sqlite3_finalize(ppStmt);
     if (result != SQLITE_OK)
     {
-        // TODO: Something is wrong... throw an error
+        throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+    }
+}
+
+void obelisk::Verb::insertVerb(sqlite3* dbConnection)
+{
+    if (dbConnection == nullptr)
+    {
+        throw obelisk::DatabaseException("database isn't open");
     }
 
-    return 0;
+    sqlite3_stmt* ppStmt = nullptr;
+
+    auto result = sqlite3_prepare_v2(dbConnection,
+        "INSERT INTO verb (name) VALUES (?)",
+        -1,
+        &ppStmt,
+        nullptr);
+    if (result != SQLITE_OK)
+    {
+        throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+    }
+
+    result
+        = sqlite3_bind_text(ppStmt, 1, getName().c_str(), -1, SQLITE_TRANSIENT);
+    switch (result)
+    {
+        case SQLITE_OK :
+            break;
+        case SQLITE_TOOBIG :
+            throw obelisk::DatabaseException::SizeException();
+            break;
+        case SQLITE_RANGE :
+            throw obelisk::DatabaseException::RangeException();
+            break;
+        case SQLITE_NOMEM :
+            throw obelisk::DatabaseException::MemoryException();
+            break;
+        default :
+            throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+            break;
+    }
+
+    result = sqlite3_step(ppStmt);
+    switch (result)
+    {
+        case SQLITE_DONE :
+            setId((int) sqlite3_last_insert_rowid(dbConnection));
+            sqlite3_set_last_insert_rowid(dbConnection, 0);
+            break;
+        case SQLITE_CONSTRAINT :
+            throw obelisk::DatabaseException::ConstraintException(
+                sqlite3_errmsg(dbConnection));
+        case SQLITE_BUSY :
+            throw obelisk::DatabaseException::BusyException();
+            break;
+        case SQLITE_MISUSE :
+            throw obelisk::DatabaseException::MisuseException();
+            break;
+        default :
+            throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+            break;
+    }
+
+    result = sqlite3_finalize(ppStmt);
+    if (result != SQLITE_OK)
+    {
+        throw obelisk::DatabaseException(sqlite3_errmsg(dbConnection));
+    }
 }
 
 int& obelisk::Verb::getId()
diff --git a/src/models/verb.h b/src/models/verb.h
index cf74204..058d1d1 100644
--- a/src/models/verb.h
+++ b/src/models/verb.h
@@ -46,8 +46,8 @@ namespace obelisk
             std::string& getName();
             void setName(std::string name);
 
-            int selectVerb(sqlite3* dbConnection);
-            int insertVerb(sqlite3* dbConnection);
+            void selectVerb(sqlite3* dbConnection);
+            void insertVerb(sqlite3* dbConnection);
     };
 } // namespace obelisk