How can I filter within collection-link? e.g. filter[collection.field]=value

if you are using the shipped sqlite backend (and no external DB like mongo) you are not able to filter by a collection link. The SQLite implementation uses GitHub - agentejo/mongo-lite: Schemaless database on top of SQLite which is not able to filter by field subsets

Filtering on a collectionlink - #9 by serjoscha87