Merge requests for relationships

[ptbrowne]

In Banks we fetch a big list of transactions and we want them to be hydrated with their bills.

client.find(TRANSACTION_DOCTYPE).include([BILL_DOCTYPE])

At the moment when relationships are fetch, they are not merged : for each transaction, one request is sent.

https://github.com/cozy/cozy-client/blob/717b22005c6ad86fbd16c871e35f80811def3b21/packages/cozy-client/src/CozyClient.js#L222-L224

https://github.com/cozy/cozy-client/blob/717b22005c6ad86fbd16c871e35f80811def3b21/packages/cozy-client/src/CozyClient.js#L267-L274

Instead of mapping directly to requests, building a (potentially big) $or request and sending 1 request should greatly reduce the number of requests (n -> 1).

Questions :

• how does the stack react to a big $or condition on ids ?
• does it limit the number of results ? If the number of ids in the $or condition is > 100, will it send all the results ? should we split the $or requests into batches of 100 ? @nono

Possible plan :

• Association should be able to return the mango query (?) to fetch included documents, without sending the query
• Client should be able to merge mango queries (?)

[nono]

If you want to fetch some documents from their IDs, you can call POST /data/:doctype/_all_docs with a keys parameter in the body. The stack doesn't add a limit on the number of fetched documents on this endpoint, but if you have a very large list, if could be better to paginate with skip and limit query-string parameters by batches of 100.