Add QgsExpression utility function to match an expression or field na…

…me string to a layer's field index
qgis · Aug 11, 2021 · 4d5014d · 4d5014d
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diff --git a/python/core/auto_generated/expression/qgsexpression.sip.in b/python/core/auto_generated/expression/qgsexpression.sip.in
@@ -286,7 +286,24 @@ Sets evaluation error (used internally by evaluation functions)
 %Docstring
 Checks whether an expression consists only of a single field reference
 
+.. seealso:: :py:func:`expressionToLayerFieldIndex`
+
 .. versionadded:: 2.9
+%End
+
+    static int expressionToLayerFieldIndex( const QString &expression, const QgsVectorLayer *layer );
+%Docstring
+Attempts to resolve an expression to a field index from the given ``layer``.
+
+Given a string which may either directly match a field name from a layer, OR may
+be an expression which consists only of a single field reference for that layer, this
+method will return the corresponding field index.
+
+:return: field index if found, or -1 if ``expression`` does not represent a field from the layer.
+
+.. seealso:: :py:func:`isField`
+
+.. versionadded:: 3.22
 %End
 
     static bool checkExpression( const QString &text, const QgsExpressionContext *context, QString &errorMessage /Out/ );

diff --git a/src/core/expression/qgsexpression.cpp b/src/core/expression/qgsexpression.cpp
@@ -1338,6 +1338,26 @@ bool QgsExpression::isField() const
   return d->mRootNode && d->mRootNode->nodeType() == QgsExpressionNode::ntColumnRef;
 }
 
+int QgsExpression::expressionToLayerFieldIndex( const QString &expression, const QgsVectorLayer *layer )
+{
+  if ( !layer )
+    return -1;
+
+  // easy check first -- lookup field directly.
+  int attrIndex = layer->fields().lookupField( expression.trimmed() );
+  if ( attrIndex >= 0 )
+    return attrIndex;
+
+  // may still be a simple field expression, just one which is enclosed in "" or similar
+  QgsExpression candidate( expression );
+  if ( candidate.isField() )
+  {
+    const QString fieldName =  qgis::down_cast<const QgsExpressionNodeColumnRef *>( candidate.rootNode() )->name();
+    return layer->fields().lookupField( fieldName );
+  }
+  return -1;
+}
+
 QList<const QgsExpressionNode *> QgsExpression::nodes() const
 {
   if ( !d->mRootNode )

diff --git a/src/core/expression/qgsexpression.h b/src/core/expression/qgsexpression.h
@@ -349,10 +349,26 @@ class CORE_EXPORT QgsExpression
 
     /**
      * Checks whether an expression consists only of a single field reference
+     *
+     * \see expressionToLayerFieldIndex()
      * \since QGIS 2.9
      */
     bool isField() const;
 
+    /**
+     * Attempts to resolve an expression to a field index from the given \a layer.
+     *
+     * Given a string which may either directly match a field name from a layer, OR may
+     * be an expression which consists only of a single field reference for that layer, this
+     * method will return the corresponding field index.
+     *
+     * \returns field index if found, or -1 if \a expression does not represent a field from the layer.
+     *
+     * \see isField()
+     * \since QGIS 3.22
+     */
+    static int expressionToLayerFieldIndex( const QString &expression, const QgsVectorLayer *layer );
+
     /**
      * Tests whether a string is a valid expression.
      * \param text string to test

diff --git a/tests/src/core/testqgsexpression.cpp b/tests/src/core/testqgsexpression.cpp
@@ -3806,6 +3806,32 @@ class TestQgsExpression: public QObject
       QCOMPARE( QgsExpression( "foo + bar" ).isField(), false );
     }
 
+    void test_expressionToLayerFieldIndex()
+    {
+      std::unique_ptr layer = std::make_unique< QgsVectorLayer >( QStringLiteral( "Point" ), QStringLiteral( "test" ), QStringLiteral( "memory" ) );
+      layer->dataProvider()->addAttributes( { QgsField( QStringLiteral( "field1" ), QVariant::String ),
+                                              QgsField( QStringLiteral( "another FIELD" ), QVariant::String ) } );
+      layer->updateFields();
+
+      QCOMPARE( QgsExpression::expressionToLayerFieldIndex( "", layer.get() ), -1 );
+      QCOMPARE( QgsExpression::expressionToLayerFieldIndex( "42", layer.get() ), -1 );
+      QCOMPARE( QgsExpression::expressionToLayerFieldIndex( "foo", layer.get() ), -1 );
+      QCOMPARE( QgsExpression::expressionToLayerFieldIndex( "\"foo bar\"", layer.get() ), -1 );
+      QCOMPARE( QgsExpression::expressionToLayerFieldIndex( "sqrt(foo)", layer.get() ), -1 );
+      QCOMPARE( QgsExpression::expressionToLayerFieldIndex( "foo + bar", layer.get() ), -1 );
+      QCOMPARE( QgsExpression::expressionToLayerFieldIndex( "field1", layer.get() ), 0 );
+      QCOMPARE( QgsExpression::expressionToLayerFieldIndex( "FIELD1", layer.get() ), 0 );
+      QCOMPARE( QgsExpression::expressionToLayerFieldIndex( "\"field1\"", layer.get() ), 0 );
+      QCOMPARE( QgsExpression::expressionToLayerFieldIndex( "\"FIELD1\"", layer.get() ), 0 );
+      QCOMPARE( QgsExpression::expressionToLayerFieldIndex( "  (  \"field1\"   )   ", layer.get() ), 0 );
+      QCOMPARE( QgsExpression::expressionToLayerFieldIndex( "another FIELD", layer.get() ), 1 );
+      QCOMPARE( QgsExpression::expressionToLayerFieldIndex( "ANOTHER field", layer.get() ), 1 );
+      QCOMPARE( QgsExpression::expressionToLayerFieldIndex( "  ANOTHER field  ", layer.get() ), 1 );
+      QCOMPARE( QgsExpression::expressionToLayerFieldIndex( "\"another field\"", layer.get() ), 1 );
+      QCOMPARE( QgsExpression::expressionToLayerFieldIndex( "\"ANOTHER FIELD\"", layer.get() ), 1 );
+      QCOMPARE( QgsExpression::expressionToLayerFieldIndex( "  (  \"ANOTHER FIELD\"   )   ", layer.get() ), 1 );
+    }
+
     void test_implicitSharing()
     {
       QgsExpression *exp = new QgsExpression( QStringLiteral( "Pilots > 2" ) );