Joins and subqueries

Try querying for an event document and return the venue reference.

*[_type == "event" && defined(venue)][0]{ 
  venue
}

The venue attribute will return a reference to another document's ID in the attribute _ref.

You can “resolve” this reference with this operator ->

This deceptively simple operator will perform a “sub-query” to resolve the document with that _id.

*[_type == "event" && defined(venue)][0]{ 
  venue->
}

Excellent! We now have the actual details of the category document. But again, we have the “too much data” problem. This can be fixed by adding an additional projection.

*[_type == "event" && defined(venue)][0]{ 
  venue->{
    name
  }
}

It’s possible to create your own subquery anywhere in a document. However, you should be mindful that additional nesting and queries can increase response time and the volume of data. Exercise caution.

References are one-way in that they are stored on a document. However the references() GROQ function allows you to look up all other references to a document ID.

A useful sub-query for this project would be to look up every event when querying for an artist, like a reverse lookup of references.

*[_type == "artist"][0]{ 
  name,
  "events": *[_type == "event" && references(^._id)]{
    name    
  }
}

The ^ character traverses out of the current filter to access the "parent." In the above example, it is accessing the _id attribute of the artist document.