Allow changing columns when foreign keys are enabled and used

When foreign keys are enabled it used to be impossible to change a
column of a table which is referenced in a foreign key. This is because
the table gets dropped in the process of renaming the column and only
then replaced by a new version.
With this commit the foreign keys are disabled for this short period of
time where the table doesn't exist and the old settings value is
restored as soon as the renaming is done.

See issue #362.

src/sqlitedb.cpp

@@ -963,6 +963,10 @@ bool DBBrowserDB::renameColumn(const QString& tablename, const QString& name, sq
             otherObjectsSql += (*it).getsql() + "\n";
     }
 
+    // Store the current foreign key settings and then disable the foreign keys being enforced to make sure the table can be dropped without errors
+    QString foreignKeysOldSettings = getPragma("foreign_keys");
+    setPragma("foreign_keys", "0");
+
     // Delete the old table
     if(!executeSQL(QString("DROP TABLE `%1`;").arg(tablename)))
     {
@@ -980,6 +984,9 @@ bool DBBrowserDB::renameColumn(const QString& tablename, const QString& name, sq
         return false;
     }
 
+    // Restore the former foreign key settings
+    setPragma("foreign_keys", foreignKeysOldSettings);
+
     // Restore the saved triggers, views and indices
     if(!executeMultiSQL(otherObjectsSql, true, true))
     {