//生成复合集合成员虚拟数据源连接。
private ISourceFragment GenerateVirtualCompositeJoinForCollectionMember(GenerateContext context, DbCollectionMemberExpression content, SelectFragment body, bool iscomposite)
{
var relation = CreateSource(context, content.RelationSet);
body.AddSource(relation);
relation.Join = EJoinType.InnerJoin;
var pairs = iscomposite ? content.CompositePairs : content.Pairs;
relation.Condition = pairs.Select(a => body.CreateExpression(a)).Merge();
return(relation);
}