mongoose 怎么 populate 别的集合的子文档?

I can't find it with populate, o.B is a objectId,o.B.name is undefined.

BSchema is a subdocument of ASchema, CSchema has a ref connection of BSchema.

I want to find all CSchema which CSchema's name is "c_xxx", and shows CSchema's B's name and its A's name.

Like this result: {CName:"c_xxx",BName:"b_xxx",AName:"a_xxx"}