Add a aggregation method to concatenate unique values

python/core/auto_generated/qgsaggregatecalculator.sip.in

@@ -56,7 +56,8 @@ to a data provider for remote calculation.
       StringMaximumLength,
       StringConcatenate,
       GeometryCollect,
-      ArrayAggregate
+      ArrayAggregate,
+      StringConcatenateUnique
     };
 
     struct AggregateParameters

src/core/qgsaggregatecalculator.cpp

@@ -163,6 +163,8 @@ QgsAggregateCalculator::Aggregate QgsAggregateCalculator::stringToAggregate( con
     return StringMaximumLength;
   else if ( normalized == QLatin1String( "concatenate" ) )
     return StringConcatenate;
+  else if ( normalized == QLatin1String( "concatenate_unique" ) )
+    return StringConcatenateUnique;
   else if ( normalized == QLatin1String( "collect" ) )
     return GeometryCollect;
   else if ( normalized == QLatin1String( "array_agg" ) )
@@ -470,6 +472,13 @@ QVariant QgsAggregateCalculator::calculate( QgsAggregateCalculator::Aggregate ag
           *ok = true;
         return concatenateStrings( fit, attr, expression, context, delimiter );
       }
+      else if ( aggregate == StringConcatenateUnique )
+      {
+        //special case
+        if ( ok )
+          *ok = true;
+        return concatenateStrings( fit, attr, expression, context, delimiter, true );
+      }
 
       bool statOk = false;
       QgsStringStatisticalSummary::Statistic stat = stringStatFromAggregate( aggregate, &statOk );
@@ -529,6 +538,7 @@ QgsStatisticalSummary::Statistic QgsAggregateCalculator::numericStatFromAggregat
     case StringMinimumLength:
     case StringMaximumLength:
     case StringConcatenate:
+    case StringConcatenateUnique:
     case GeometryCollect:
     case ArrayAggregate:
     {
@@ -577,6 +587,7 @@ QgsStringStatisticalSummary::Statistic QgsAggregateCalculator::stringStatFromAgg
     case ThirdQuartile:
     case InterQuartileRange:
     case StringConcatenate:
+    case StringConcatenateUnique:
     case GeometryCollect:
     case ArrayAggregate:
     {
@@ -624,6 +635,7 @@ QgsDateTimeStatisticalSummary::Statistic QgsAggregateCalculator::dateTimeStatFro
     case StringMinimumLength:
     case StringMaximumLength:
     case StringConcatenate:
+    case StringConcatenateUnique:
     case GeometryCollect:
     case ArrayAggregate:
     {
@@ -712,30 +724,32 @@ QVariant QgsAggregateCalculator::calculateGeometryAggregate( QgsFeatureIterator
 }
 
 QVariant QgsAggregateCalculator::concatenateStrings( QgsFeatureIterator &fit, int attr, QgsExpression *expression,
-    QgsExpressionContext *context, const QString &delimiter )
+    QgsExpressionContext *context, const QString &delimiter, bool unique )
 {
   Q_ASSERT( expression || attr >= 0 );
 
   QgsFeature f;
-  QString result;
+  QStringList results;
   while ( fit.nextFeature( f ) )
   {
-    if ( !result.isEmpty() )
-      result += delimiter;
-
+    QString result;
     if ( expression )
     {
       Q_ASSERT( context );
       context->setFeature( f );
       QVariant v = expression->evaluate( context );
-      result += v.toString();
+      result = v.toString();
     }
     else
     {
-      result += f.attribute( attr ).toString();
+      result = f.attribute( attr ).toString();
     }
+
+    if ( !unique || !results.contains( result ) )
+      results << result;
   }
-  return result;
+
+  return results.join( delimiter );
 }
 
 QVariant QgsAggregateCalculator::defaultValue( QgsAggregateCalculator::Aggregate aggregate ) const
@@ -750,6 +764,7 @@ QVariant QgsAggregateCalculator::defaultValue( QgsAggregateCalculator::Aggregate
       return 0;
 
     case StringConcatenate:
+    case StringConcatenateUnique:
       return ""; // zero length string - not null!
 
     case ArrayAggregate:

src/core/qgsaggregatecalculator.h

@@ -81,7 +81,8 @@ class CORE_EXPORT QgsAggregateCalculator
       StringMaximumLength, //!< Maximum length of string (string fields only)
       StringConcatenate, //!< Concatenate values with a joining string (string fields only). Specify the delimiter using setDelimiter().
       GeometryCollect, //!< Create a multipart geometry from aggregated geometries
-      ArrayAggregate //!< Create an array of values
+      ArrayAggregate, //!< Create an array of values
+      StringConcatenateUnique //!< Concatenate unique values with a joining string (string fields only). Specify the delimiter using setDelimiter().
     };
 
     //! A bundle of parameters controlling aggregate calculation
@@ -207,7 +208,7 @@ class CORE_EXPORT QgsAggregateCalculator
                                const QString &delimiter,
                                QgsExpressionContext *context, bool *ok = nullptr );
     static QVariant concatenateStrings( QgsFeatureIterator &fit, int attr, QgsExpression *expression,
-                                        QgsExpressionContext *context, const QString &delimiter );
+                                        QgsExpressionContext *context, const QString &delimiter, bool unique = false );
 
     QVariant defaultValue( Aggregate aggregate ) const;
 };

tests/src/python/test_qgsaggregatecalculator.py

@@ -188,6 +188,9 @@ def testString(self):
         val, ok = agg.calculate(QgsAggregateCalculator.StringConcatenate, 'fldstring')
         self.assertTrue(ok)
         self.assertEqual(val, 'cc,aaaa,bbbbbbbb,aaaa,eeee,,eeee,,dddd')
+        val, ok = agg.calculate(QgsAggregateCalculator.StringConcatenateUnique, 'fldstring')
+        self.assertTrue(ok)
+        self.assertEqual(val, 'cc,aaaa,bbbbbbbb,eeee,,dddd')
 
         # bad tests - the following stats should not be calculatable for string fields
         for t in [QgsAggregateCalculator.Sum,
@@ -458,6 +461,7 @@ def testStringToAggregate(self):
                  [QgsAggregateCalculator.StringMinimumLength, 'min_length'],
                  [QgsAggregateCalculator.StringMaximumLength, 'max_length'],
                  [QgsAggregateCalculator.StringConcatenate, 'concatenate'],
+                 [QgsAggregateCalculator.StringConcatenateUnique, 'concatenate_unique'],
                  [QgsAggregateCalculator.GeometryCollect, 'collect']]
 
         for t in tests:

tests/src/python/test_qgsvectorlayer.py

@@ -2074,7 +2074,7 @@ def testAggregate(self):
         layer = QgsVectorLayer("Point?field=fldstring:string", "layer", "memory")
         pr = layer.dataProvider()
 
-        string_values = ['this', 'is', 'a', 'test']
+        string_values = ['this', 'is', 'a', 'test', 'a', 'nice', 'test']
         features = []
         for s in string_values:
             f = QgsFeature()
@@ -2086,7 +2086,10 @@ def testAggregate(self):
         params.delimiter = ' '
         val, ok = layer.aggregate(QgsAggregateCalculator.StringConcatenate, 'fldstring', params)
         self.assertTrue(ok)
-        self.assertEqual(val, 'this is a test')
+        self.assertEqual(val, 'this is a test a nice test')
+        val, ok = layer.aggregate(QgsAggregateCalculator.StringConcatenateUnique, 'fldstring', params)
+        self.assertTrue(ok)
+        self.assertEqual(val, 'this is a test nice')
 
     def testAggregateInVirtualField(self):
         """