Get parent via child without parent id

I am new to elasticsearch, but can not find the information about one issue which is:

Imagine:

You have Entity A and it has mapping to Entity B (for example ManyToOne), but Entity B has no mapping to Entity A. I am speaking here about Relationship mapping annotations.

Basically A knows about B, but B does not know about A. How to then search something via ElasticSearch when you have some part of entity B and you would like to find entity A by this part from B?

Do I have to provide mapping in entity B to A, or there is a way to find by entity B, relation to entity A.

I hope I explained my concern clearly...

Regards