Add QgsProject::transactionGroup() available & with Python bindings.

src/core/qgsproject.cpp

@@ -2574,6 +2574,10 @@ QMap<QString, QgsMapLayer *> QgsProject::mapLayers() const
   return mLayerStore->mapLayers();
 }
 
+QgsTransactionGroup *QgsProject::transactionGroup( const QString &providerKey, const QString &connString )
+{
+  return mTransactionGroups.value( qMakePair( providerKey, connString ) );
+}
 
 QgsCoordinateReferenceSystem QgsProject::defaultCrsForNewLayers() const
 {

src/core/qgsproject.h

@@ -584,14 +584,23 @@ class CORE_EXPORT QgsProject : public QObject, public QgsExpressionContextGenera
      */
     QMap< QPair< QString, QString>, QgsTransactionGroup *> transactionGroups() SIP_SKIP;
 
+    /**
+     * Get a transaction group by getting provider & connection string.
+     * Returns null pointer if transaction group is not available.
+     *
+     * providerKey, connString -> transactionGroup
+     *
+     * \since QGIS 3.0
+     */
+    QgsTransactionGroup *transactionGroup( const QString &providerKey, const QString &connString );
+
     /**
      * Should default values be evaluated on provider side when requested and not when committed.
      *
      * \since QGIS 2.16
      */
     bool evaluateDefaultValues() const;
 
-
     /**
      * Defines if default values should be evaluated on provider side when requested and not when committed.
      *

tests/src/python/Testing/Temporary/CTestCostData.txt

@@ -0,0 +1 @@
+---

tests/src/python/test_qgsproject.py

@@ -54,6 +54,11 @@ def __init__(self, methodName):
         unittest.TestCase.__init__(self, methodName)
         self.messageCaught = False
 
+    def setUpClass(cls):
+        cls.dbconn = 'dbname=\'qgis_test\''
+        if 'QGIS_PGTEST_DB' in os.environ:
+            cls.dbconn = os.environ['QGIS_PGTEST_DB']
+
     def test_makeKeyTokens_(self):
         # see http://www.w3.org/TR/REC-xml/#d0e804 for a list of valid characters
 
@@ -689,6 +694,17 @@ def testTakeLayer(self):
         p = None
         self.assertTrue(l1.isValid())
 
+    def test_transactionsGroup(self):
+        # No transaction group.
+        QgsProject.instance().setAutoTransaction(False)
+        noTg = QgsProject.instance().transactionGroup("provider-key", "database-connection-string")
+        self.assertIsNone(noTg)
+
+        # Undefined transaction group (wrong provider key).
+        QgsProject.instance().setAutoTransaction(True)
+        noTg = QgsProject.instance().transactionGroup("provider-key", "database-connection-string")
+        self.assertIsNone(noTg)
+
     def test_zip_new_project(self):
         tmpDir = QTemporaryDir()
         tmpFile = "{}/project.qgz".format(tmpDir.path())