Relations

A common use case for field resolvers is fetching one or more item associated with the current document, such as the user corresponding to a post's userId field; or the array of categories correponding to its categoriesIds field.

While you can achieve this by explicitly adding an API schema field, Vulcan also offers a shortcut syntax using the relation property:

userId: {
  type: String,
  optional: true,
  canRead: ['guests'],
  relation: {
    fieldName: 'user',
    typeName: 'User',
    kind: 'hasOne', // one of 'hasOne' or 'hasMany'
  }
},

Since this is a hasOne relation, Vulcan will assume that the current field stores the _id of the related document to look up; and because we know that the type of the returned item should be User we can also easily figure out that we need to search the Users collection.

Similarly, you can also define hasMany relations:

categoriesIds: {
  type: Array,
  arrayItem: {
    type: String,
    optional: true,
    canRead: ['guests'],
  }
  optional: true,
  canRead: ['guests'],
  relation: {
    fieldName: 'categories',
    typeName: '[Categories]',
    kind: 'hasMany'
  }
},

Relations, Cards, and Datatables

As an added bonus, if you define the userId field as having a relation to the User type, Cards and Datatables will be able to understand this relation and use smarter formatting when displaying this data.