Writing Schemas

A GraphQL API starts by building a schema. Using Absinthe, schemas are normal modules that use Absinthe.Schema.

Here's a schema that supports looking up an item by ID:

# filename: myapp/schema.ex
defmodule MyAppWeb.Schema do

  use Absinthe.Schema

  # Example data
  @items %{
    "foo" => %{id: "foo", name: "Foo"},
    "bar" => %{id: "bar", name: "Bar"}
  }

  query do
    field :item, :item do
      arg :id, non_null(:id)
      resolve fn %{id: item_id}, _ ->
        {:ok, @items[item_id]}
      end
    end
  end

end

You may want to refer to the Absinthe API documentation for more detailed information as you look this over..

Some macros and functions used here that are worth mentioning, pulled in automatically from Absinthe.Schema.Notation by use Absinthe.Schema:

• query - Defines the root query object. It's like using object but with nice defaults. There is a matching mutation macro as well.
• field - Defines a field in the enclosing object, input_object, or interface.
• arg - Defines an argument in the enclosing field or directive.
• resolve - Sets the resolve function for the enclosing field.

You'll notice we mention some types being referenced: :item and :id. :id is a built-in scalar type (like :string, :boolean, and others), but :item we need to define ourselves.

We can do it in the same MyAppWeb.Schema module, using the object macro defined by Absinthe.Schema.Notation:

# filename: myapp/schema.ex
@desc "An item"
object :item do
  field :id, :id
  field :name, :string
end

Now you can use Absinthe to execute a query document. Keep in mind that for HTTP, you'll probably want to use Absinthe.Plug instead of executing GraphQL query documents yourself. Absinthe doesn't know or care about HTTP, but the absinthe_plug project does: it handles the vagaries of interacting with HTTP GraphQL clients so you don't have to.

If you were executing query documents yourself (let's assume for a local tool), it would go something like this:

"""
{
  item(id: "foo") {
    name
  }
}
"""
|> Absinthe.run(MyAppWeb.Schema)

# Result
{:ok, %{data: %{"item" => %{"name" => "Foo"}}}}

Your schemas can be further customized using the options available to Absinthe.Schema.Notation.field/4 to help provide for a richer experience for your users, customize the field names, or mark fields as deprecated.

# filename: myapp/language_schema.ex
@desc "A Language"
object :language do
  field :id, :id
  field :iso_639_1, :string, description: "2 character ISO 639-1 code", name: "iso639"
  field :name, :string, description: "English name of the language"
end

Importing Types

We could also move our type definitions out into a different module, for instance, MyAppWeb.Schema.Types, and then use import_types in our MyAppWeb.Schema:

# filename: myapp/schema/types.ex
defmodule MyAppWeb.Schema.Types do
  use Absinthe.Schema.Notation

  object :item do
    field :id, :id
    field :name, :string
  end

  # ...

end

# filename: myapp/schema.ex
defmodule MyAppWeb.Schema do
  use Absinthe.Schema

  import_types MyAppWeb.Schema.Types

  # ...

end

It's a nice way of separating the top-level query and mutation information, which define the surface area of the API, with the actual types that it uses.

See Importing Types for a full guide to importing types.