Allow maximum number of entries to show in relation reference widget

to be configurable via an advanced setting configuration option

In some circumstances 100 is not enough

(cherry picked from commit a97ce69)

Author: nyalldawson

resources/qgis_global_settings.ini

@@ -77,3 +77,9 @@ helpSearchPath=https://docs.qgis.org/$qgis_short_version/$qgis_locale/docs/user_
 [providers]
 # Default timeout for PostgreSQL servers (seconds)
 PostgreSQL\default_timeout=30
+
+[gui]
+# Maximum number of entries to show in Relation Reference widgets. Too large a number here can
+# cause performance issues, as the records must all be loaded from the related table on display.
+maxEntriesRelationWidget=100
+

src/core/qgsfeaturefiltermodel.cpp

@@ -18,6 +18,7 @@
 
 #include "qgsvectorlayer.h"
 #include "qgsconditionalstyle.h"
+#include "qgssettings.h"
 
 QgsFeatureFilterModel::QgsFeatureFilterModel( QObject *parent )
   : QAbstractItemModel( parent )
@@ -357,7 +358,7 @@ void QgsFeatureFilterModel::scheduledReload()
   request.setSubsetOfAttributes( attributes, mSourceLayer->fields() );
   request.setFlags( QgsFeatureRequest::NoGeometry );
 
-  request.setLimit( 100 );
+  request.setLimit( QgsSettings().value( QStringLiteral( "maxEntriesRelationWidget" ), 100, QgsSettings::Gui ).toInt() );
 
   mGatherer = new QgsFieldExpressionValuesGatherer( mSourceLayer, mDisplayExpression, mIdentifierField, request );
   mGatherer->setData( mShouldReloadCurrentFeature );