Add support for in-memory databases

Add a new menu option for opening an in-memory database. When working on
an in-memory database some menu options are disabled because they only
make sense when the database is also written to disk. I have left the
Write Changes button enabled though because for some actions the current
transaction needs to be committed - even though it's only committed to
memory then.

See issues #335 and #1492.

src/MainWindow.cpp

@@ -433,6 +433,19 @@ void MainWindow::fileNew()
     }
 }
 
+void MainWindow::fileNewInMemoryDatabase()
+{
+    db.create(":memory:");
+    setCurrentFile(tr("In-Memory database"));
+    statusEncodingLabel->setText(db.getPragma("encoding"));
+    statusEncryptionLabel->setVisible(false);
+    statusReadOnlyLabel->setVisible(false);
+    loadExtensionsFromSettings();
+    populateTable();
+    openSqlTab(true);
+    createTable();
+}
+
 void MainWindow::populateStructure(const QString& old_table)
 {
     // Refresh the structure tab
@@ -1714,6 +1727,7 @@ void MainWindow::dropEvent(QDropEvent *event)
 void MainWindow::activateFields(bool enable)
 {
     bool write = !db.readOnly();
+    bool tempDb = db.currentFile() == ":memory:";
 
     ui->fileCloseAction->setEnabled(enable);
     ui->fileAttachAction->setEnabled(enable);
@@ -1736,8 +1750,8 @@ void MainWindow::activateFields(bool enable)
     ui->actionExecuteSql->setEnabled(enable);
     ui->actionLoadExtension->setEnabled(enable);
     ui->actionSqlExecuteLine->setEnabled(enable);
-    ui->actionSaveProject->setEnabled(enable);
-    ui->actionEncryption->setEnabled(enable && write);
+    ui->actionSaveProject->setEnabled(enable && !tempDb);
+    ui->actionEncryption->setEnabled(enable && write && !tempDb);
     ui->buttonClearFilters->setEnabled(enable);
     ui->buttonSaveFilterAsPopup->setEnabled(enable);
     ui->dockEdit->setEnabled(enable);

src/MainWindow.h

@@ -212,6 +212,7 @@ private slots:
     void createTreeContextMenu(const QPoint & qPoint);
     void changeTreeSelection();
     void fileNew();
+    void fileNewInMemoryDatabase();
     void populateTable();
     void clearTableBrowser();
     bool fileClose();

src/MainWindow.ui

@@ -910,6 +910,7 @@ You can drag SQL statements from an object row and drop them into other applicat
      <addaction name="fileExportJsonAction"/>
     </widget>
     <addaction name="fileNewAction"/>
+    <addaction name="fileNewInMemoryDatabaseAction"/>
     <addaction name="fileOpenAction"/>
     <addaction name="fileOpenReadOnlyAction"/>
     <addaction name="fileAttachAction"/>
@@ -2093,6 +2094,11 @@ You can drag SQL statements from the Schema column and drop them into the SQL ed
     <enum>QAction::TextHeuristicRole</enum>
    </property>
   </action>
+  <action name="fileNewInMemoryDatabaseAction">
+   <property name="text">
+    <string>New In-&amp;Memory Database</string>
+   </property>
+  </action>
  </widget>
  <customwidgets>
   <customwidget>
@@ -3401,6 +3407,22 @@ You can drag SQL statements from the Schema column and drop them into the SQL ed
     </hint>
    </hints>
   </connection>
+  <connection>
+   <sender>fileNewInMemoryDatabaseAction</sender>
+   <signal>triggered()</signal>
+   <receiver>MainWindow</receiver>
+   <slot>fileNewInMemoryDatabase()</slot>
+   <hints>
+    <hint type="sourcelabel">
+     <x>518</x>
+     <y>314</y>
+    </hint>
+    <hint type="destinationlabel">
+     <x>518</x>
+     <y>314</y>
+    </hint>
+   </hints>
+  </connection>
  </connections>
  <slots>
   <slot>fileOpen()</slot>
@@ -3470,5 +3492,6 @@ You can drag SQL statements from the Schema column and drop them into the SQL ed
   <slot>changeSqlTab(int)</slot>
   <slot>hideColumns()</slot>
   <slot>renameSqlTab(int)</slot>
+  <slot>fileNewInMemoryDatabase()</slot>
  </slots>
 </ui>

src/sqlitedb.cpp

@@ -529,11 +529,22 @@ bool DBBrowserDB::close()
     {
         if (getDirty())
         {
-            QMessageBox::StandardButton reply = QMessageBox::question(nullptr,
-                                                                      QApplication::applicationName(),
-                                                                      tr("Do you want to save the changes "
-                                                                         "made to the database file %1?").arg(curDBFilename),
-                                                                      QMessageBox::Save | QMessageBox::No | QMessageBox::Cancel);
+            // In-memory databases can't be saved to disk. So the need another text than regular databases.
+            // Note that the QMessageBox::Yes option in the :memory: case and the QMessageBox::No option in the regular case are
+            // doing the same job: proceeding but not saving anything.
+            QMessageBox::StandardButton reply;
+            if(curDBFilename == ":memory:")
+            {
+                reply = QMessageBox::question(nullptr,
+                                              QApplication::applicationName(),
+                                              tr("Do you really want to close this temporary database? All data will be lost."),
+                                              QMessageBox::Yes | QMessageBox::Cancel);
+            } else {
+                reply = QMessageBox::question(nullptr,
+                                              QApplication::applicationName(),
+                                              tr("Do you want to save the changes made to the database file %1?").arg(curDBFilename),
+                                              QMessageBox::Save | QMessageBox::No | QMessageBox::Cancel);
+            }
 
             // If the user clicked the cancel button stop here and return false
             if(reply == QMessageBox::Cancel)