What is the join logic when using API chaining with action = get?

Question

I read about API chaining and I'm using it with get actions to learn more about how it works. I'm confused by the inconsistent behavior of the following two API calls:

---

CRM.api3('participant', 'get', 
  {"id": 12345, "api.Event.get": {} });

That gets a participant record and embeds the single associated event record within the results. Cool.

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But this one works differently:

CRM.api3('participant', 'get', 
  {"id": 12345, "api.ParticipantStatusType.get": {} });

It gets the same participant record, but embeds all status type records within the results. Why?

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In general, I'd like to understand what logic CiviCRM is using to filter the results of chained calls.

Answer 1

Chaining will attempt to "guess" at the id of the chained entity. In your first example, the fetched participant record has an "event_id" and the api chaining controller passes that into the chained Event api call as the id.

The guessing is pretty basic though. For everything else, you should specify params explicitly. You do so by using the magic "$value" notation. E.g.

CRM.api3('participant', 'get', {
  id: 12345,
  "api.ParticipantStatusType.get": {id: '$value.status_id'}
});

Note that you can also use joins for this type of thing, which is generally more efficient than chaining. Check out joins in the Api explorer.