protected virtual Expression BindGroupBy(Expression source, LambdaExpression keySelector, LambdaExpression elementSelector, LambdaExpression resultSelector)
{
ProjectionExpression projection = this.VisitSequence(source);
this.map[keySelector.Parameters[0]] = projection.Projector;
Expression keyExpr = this.Visit(keySelector.Body);
Expression elemExpr = projection.Projector;
if (elementSelector != null)
{
this.map[elementSelector.Parameters[0]] = projection.Projector;
elemExpr = this.Visit(elementSelector.Body);
}
// Use ProjectColumns to get group-by expressions from key expression
ProjectedColumns keyProjection = this.ProjectColumns(keyExpr, projection.Source.Alias, projection.Source.Alias);
IEnumerable <Expression> groupExprs = keyProjection.Columns.Select(c => c.Expression);
// make duplicate of source query as basis of element subquery by visiting the source again
ProjectionExpression subqueryBasis = this.VisitSequence(source);
// recompute key columns for group expressions relative to subquery (need these for doing the correlation predicate)
this.map[keySelector.Parameters[0]] = subqueryBasis.Projector;
Expression subqueryKey = this.Visit(keySelector.Body);
// use same projection trick to get group-by expressions based on subquery
ProjectedColumns subqueryKeyPC = this.ProjectColumns(subqueryKey, subqueryBasis.Source.Alias, subqueryBasis.Source.Alias);
IEnumerable <Expression> subqueryGroupExprs = subqueryKeyPC.Columns.Select(c => c.Expression);
Expression subqueryCorrelation = this.BuildPredicateWithNullsEqual(subqueryGroupExprs, groupExprs);
// compute element based on duplicated subquery
Expression subqueryElemExpr = subqueryBasis.Projector;
if (elementSelector != null)
{
this.map[elementSelector.Parameters[0]] = subqueryBasis.Projector;
subqueryElemExpr = this.Visit(elementSelector.Body);
}
// build subquery that projects the desired element
string elementAlias = this.GetNextAlias();
ProjectedColumns elementPC = this.ProjectColumns(subqueryElemExpr, elementAlias, subqueryBasis.Source.Alias);
ProjectionExpression elementSubquery =
new ProjectionExpression(
new SelectExpression(TypeSystem.GetSequenceType(subqueryElemExpr.Type), elementAlias, elementPC.Columns, subqueryBasis.Source, subqueryCorrelation),
elementPC.Projector
);
string alias = this.GetNextAlias();
// make it possible to tie aggregates back to this group-by
GroupByInfo info = new GroupByInfo(alias, elemExpr);
this.groupByMap.Add(elementSubquery, info);
Expression resultExpr;
if (resultSelector != null)
{
Expression saveGroupElement = this.currentGroupElement;
this.currentGroupElement = elementSubquery;
// compute result expression based on key & element-subquery
this.map[resultSelector.Parameters[0]] = keyProjection.Projector;
this.map[resultSelector.Parameters[1]] = elementSubquery;
resultExpr = this.Visit(resultSelector.Body);
this.currentGroupElement = saveGroupElement;
}
else
{
// result must be IGrouping<K,E>
resultExpr = Expression.New(
typeof(Grouping <,>).MakeGenericType(keyExpr.Type, subqueryElemExpr.Type).GetConstructors()[0],
new Expression[] { keyExpr, elementSubquery }
);
}
ProjectedColumns pc = this.ProjectColumns(resultExpr, alias, projection.Source.Alias);
// make it possible to tie aggregates back to this group-by
Expression projectedElementSubquery = ((NewExpression)pc.Projector).Arguments[1];
this.groupByMap.Add(projectedElementSubquery, info);
return(new ProjectionExpression(
new SelectExpression(TypeSystem.GetSequenceType(resultExpr.Type), alias, pc.Columns, projection.Source, null, null, groupExprs),
pc.Projector
));
}