Why does a `hasMany` relationship create an OPTIONS and GET request for each item in the relationship?

Question

I currently have a response like so from my API:

{
  "data": [{
      "attributes": {
        "customer_name": "The Name",
      },
      "id": 1,
      "relationships": {
        "tasks": {
          "data": [{
              "attributes": {
                "name": "The task",
              },

            ]
          }
        },
        "type": "customer"
      }
    ]
  }

And Ember loads the model correctly. However, when I try and consume the computed property the relationship creates, Ember initiates an OPTIONS and subsequent GET request for each item in the relationship.

I haven't worked with these kinds of relationships before so this may be the expected behaviour but I'm a bit confused.

If it is going to do a request for each one, why bother sending the items as part of the original model via a relationship at all? Why not just do a normal model retrieval which would probably be much quicker.

It seems really wasteful so I suspect I'm not handling or understanding something correctly, I'd appreciate any insight.

Answer 1

This is expected behavior for what you are doing (accessing a hasMany relationship). In your example, the tasks do not exist in Ember Data's store so when trying to access them Ember Data is smart enough to fetch them for you from your API. In doing so, it sends out the OPTIONS and GET requests to retrieve them. The OPTIONS request is simply a preflight request.

If you wanted to work with this particular model and its tasks (or other hasMany relationship models) without making so many network requests, you could fetch both simultaneously by requesting them with an include assuming you're using an adapter that allows it (i.e., supports the JSON API spec) and your API does too:

// for example, fetching a post with its comments (hasMany relationship)
this.store.findRecord('post', params.post_id, { include: 'comments' } );

Doing so should return the model and its tasks in one network request. For further reading, I'd checkout Ember's guide on Relationships.