Determining existence of relationship without server query

[sandstrom]

Sometimes it's useful to determine if a relationship is present without querying the server. child.get('mother') will query the server if the mother isn't present.

This private API returns the answer without querying the server !!child._relationships.mother.inverseRecord.

On knowledge of associated records
Mostly, only the 1-side in N-1 and 1-1 relationships will know whether it's associated with another record (given how foreign keys are located). The N-side won't know without querying the server.

Ways to provide this

1. child.isPresent('mother') return true/false
2. child.get('mother', { local: true }) return the record if it exists in store, otherwise null
3. child.count('mother') could return without querying the server for 1-side, and could trigger a server request for the N-side. It's main usage would obviously be remote counting, but that way the API addition would be useful for more than probing for existence.
4. eachRelationship could be amended to provide info on relationship presence (similar to the current, private, API quoted above).

I think I like (3) the most, but depends on whether others think that's a useful addition. Thoughts?

[wecc]

Relevant: #2431

[igorT]

emberjs/rfcs#57 is aiming to address this

[pangratz]

I think this issue can be closed since an initial implementation of references has been merged into master. Having the ds-references feature enabled, it can be checked if there is data for a relationship via model.belongsTo("family").value() !== null and model.hasMany("children").value() !== null.

[fivetanley]

Closing in favor of references!

[sandstrom]

If anyone comes across this issue, have a look at #7324 and #7102.

[runspired]

#7796 will add autotracking for this feature

[sandstrom]

awesome @runspired