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Abstract Type Union

Abstract Type Union

Learn how to use union and introspecting value types in GraphQL.

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Using unions

A GraphQL union type is an abstract type that represents a set of specific concrete types. For instance, in our PlateSlate search example, a :search_result could be a union type for both :menu_item and :category. Let’s define that in our schema. However, first, we need to add the :category type. It’s a straightforward grouping of :menu_item records with a name and description:

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alias PlateSlateWeb.Resolvers
object :category do
  field :name, :string
  field :description, :string
  field :items, list_of(:menu_item) do
    resolve &Resolvers.Menu.items_for_category/3
  end
end

We’ve included a resolver to load the menu items for a category when the :items field is selected. At the moment, the implementation is a bit naive:

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def items_for_category(category, _, _) do 
    query = Ecto.assoc(category, :items) 
    {:ok, PlateSlate.Repo.all(query)}
end

This is the first resolver we’ve written using the first argument which receives the parent value. In our case, this resolver is on the :items field of the :category object, so its parent value is a category. We can then use that category to do a database query for its items. Let’s define the :search_result union type in menu_types.ex:

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union :search_result do
  types [:menu_item, :category]
   #<<almost done...>>
end

This uses a couple of new macros from Absinthe. The union macro creates our type, ...