Enforce unique constraints in attribute form

python/gui/editorwidgets/core/qgseditorwidgetwrapper.sip

@@ -104,12 +104,21 @@ class QgsEditorWidgetWrapper : QgsWidgetWrapper
 
     /**
      * Get the current constraint status.
-     * @return true if the constraint is valid or if there's not constraint,
+     * @return true if the constraint is valid or if there's no constraint,
      * false otherwise
      * @note added in QGIS 2.16
+     * @see constraintFailureReason()
      */
     bool isValidConstraint() const;
 
+    /**
+     * Returns the reason why a constraint check has failed (or an empty string
+     * if constraint check was successful).
+     * @see isValidConstraint()
+     * @note added in QGIS 3.0
+     */
+    QString constraintFailureReason() const;
+
   signals:
     /**
      * Emit this signal, whenever the value changed.

src/gui/editorwidgets/core/qgseditorwidgetwrapper.cpp

@@ -17,6 +17,7 @@
 #include "qgsvectorlayer.h"
 #include "qgsvectordataprovider.h"
 #include "qgsfields.h"
+#include "qgsvectorlayerutils.h"
 
 #include <QTableView>
 
@@ -108,14 +109,19 @@ void QgsEditorWidgetWrapper::updateConstraintWidgetStatus( bool constraintValid
 void QgsEditorWidgetWrapper::updateConstraint( const QgsFeature &ft )
 {
   bool toEmit( false );
-  QString errStr( tr( "predicate is True" ) );
   QString expression = layer()->editFormConfig().constraintExpression( mFieldIdx );
-  QString description;
+  QStringList expressions, descriptions;
   QVariant value = ft.attribute( mFieldIdx );
+  QString fieldName = ft.fields().count() > mFieldIdx ? ft.fields().field( mFieldIdx ).name() : QString();
+
+  mConstraintFailureReason.clear();
+
+  QStringList errors;
 
   if ( ! expression.isEmpty() )
   {
-    description = layer()->editFormConfig().constraintDescription( mFieldIdx );
+    expressions << expression;
+    descriptions << layer()->editFormConfig().constraintDescription( mFieldIdx );
 
     QgsExpressionContext context = layer()->createExpressionContext();
     context.setFeature( ft );
@@ -125,11 +131,17 @@ void QgsEditorWidgetWrapper::updateConstraint( const QgsFeature &ft )
     mValidConstraint = expr.evaluate( &context ).toBool();
 
     if ( expr.hasParserError() )
-      errStr = expr.parserErrorString();
+    {
+      errors << tr( "Parser error: %1" ).arg( expr.parserErrorString() );
+    }
     else if ( expr.hasEvalError() )
-      errStr = expr.evalErrorString();
+    {
+      errors << tr( "Evaluation error: %1" ).arg( expr.evalErrorString() );
+    }
     else if ( ! mValidConstraint )
-      errStr = tr( "predicate is False" );
+    {
+      errors << tr( "%1 check failed" ).arg( layer()->editFormConfig().constraintDescription( mFieldIdx ) );
+    }
 
     toEmit = true;
   }
@@ -138,30 +150,66 @@ void QgsEditorWidgetWrapper::updateConstraint( const QgsFeature &ft )
 
   if ( layer()->fieldConstraints( mFieldIdx ) & QgsField::ConstraintNotNull )
   {
+    descriptions << QStringLiteral( "NotNull" );
     if ( !expression.isEmpty() )
     {
-      QString fieldName = ft.fields().field( mFieldIdx ).name();
-      expression = "( " + expression + " ) AND ( " + fieldName + " IS NOT NULL)";
-      description = "( " + description + " ) AND NotNull";
+      expressions << fieldName + " IS NOT NULL";
     }
     else
     {
-      description = QStringLiteral( "NotNull" );
-      expression = QStringLiteral( "NotNull" );
+      expressions << QStringLiteral( "NotNull" );
     }
 
     mValidConstraint = mValidConstraint && !value.isNull();
 
     if ( value.isNull() )
-      errStr = tr( "predicate is False" );
+    {
+      errors << tr( "Value is NULL" );
+    }
+
+    toEmit = true;
+  }
+
+  if ( layer()->fieldConstraints( mFieldIdx ) & QgsField::ConstraintUnique )
+  {
+    descriptions << QStringLiteral( "Unique" );
+    if ( !expression.isEmpty() )
+    {
+      expressions << fieldName + " IS UNIQUE";
+    }
+    else
+    {
+      expression = QStringLiteral( "Unique" );
+    }
+
+    bool alreadyExists = QgsVectorLayerUtils::valueExists( layer(), mFieldIdx, value, QgsFeatureIds() << ft.id() );
+    mValidConstraint = mValidConstraint && !alreadyExists;
+
+    if ( alreadyExists )
+    {
+      errors << tr( "Value is not unique" );
+    }
 
     toEmit = true;
   }
 
   if ( toEmit )
   {
+    QString errStr = errors.isEmpty() ? tr( "Constraint checks passed" ) : errors.join( '\n' );
+    mConstraintFailureReason = errors.join( ", " );
+    QString description;
+    if ( descriptions.size() > 1 )
+      description = "( " + descriptions.join( " ) AND ( " ) + " )";
+    else if ( !descriptions.isEmpty() )
+      description = descriptions.at( 0 );
+    QString expressionDesc;
+    if ( expressions.size() > 1 )
+      expressionDesc = "( " + expressions.join( " ) AND ( " ) + " )";
+    else if ( !expressions.isEmpty() )
+      expressionDesc = expressions.at( 0 );
+
     updateConstraintWidgetStatus( mValidConstraint );
-    emit constraintStatusChanged( expression, description, errStr, mValidConstraint );
+    emit constraintStatusChanged( expressionDesc, description, errStr, mValidConstraint );
   }
 }
 
@@ -170,6 +218,11 @@ bool QgsEditorWidgetWrapper::isValidConstraint() const
   return mValidConstraint;
 }
 
+QString QgsEditorWidgetWrapper::constraintFailureReason() const
+{
+  return mConstraintFailureReason;
+}
+
 bool QgsEditorWidgetWrapper::isInTable( const QWidget* parent )
 {
   if ( !parent ) return false;

src/gui/editorwidgets/core/qgseditorwidgetwrapper.h

@@ -124,12 +124,21 @@ class GUI_EXPORT QgsEditorWidgetWrapper : public QgsWidgetWrapper
 
     /**
      * Get the current constraint status.
-     * @return true if the constraint is valid or if there's not constraint,
+     * @return true if the constraint is valid or if there's no constraint,
      * false otherwise
      * @note added in QGIS 2.16
+     * @see constraintFailureReason()
      */
     bool isValidConstraint() const;
 
+    /**
+     * Returns the reason why a constraint check has failed (or an empty string
+     * if constraint check was successful).
+     * @see isValidConstraint()
+     * @note added in QGIS 3.0
+     */
+    QString constraintFailureReason() const;
+
   signals:
     /**
      * Emit this signal, whenever the value changed.
@@ -239,6 +248,9 @@ class GUI_EXPORT QgsEditorWidgetWrapper : public QgsWidgetWrapper
      */
     bool mValidConstraint;
 
+    //! Contains the string explanation of why a constraint check failed
+    QString mConstraintFailureReason;
+
     int mFieldIdx;
     QgsFeature mFeature;
     mutable QVariant mDefaultValue; // Cache default value, we don't want to retrieve different serial numbers if called repeatedly

src/gui/qgsattributeform.cpp

@@ -824,8 +824,7 @@ bool QgsAttributeForm::currentFormValidConstraints( QStringList &invalidFields,
       {
         invalidFields.append( eww->field().name() );
 
-        QString desc = eww->layer()->editFormConfig().constraintDescription( eww->fieldIdx() );
-        descriptions.append( desc );
+        descriptions.append( eww->constraintFailureReason() );
 
         valid = false; // continue to get all invalif fields
       }