Middleware enables custom resolution behaviour on a field.

All resolution happens through middleware. Even resolve functions are middleware, as the resolve macro is just

quote do
  middleware Absinthe.Resolution, unquote(function_ast)
end

Resolution happens by reducing a list of middleware spec onto an %Absinthe.Resolution{} struct.

Example

defmodule MyApp.Web.Authentication do
  @behaviour Absinthe.Middleware

  def call(resolution, _config) do
    case resolution.context do
      %{current_user: _} ->
        resolution
      _ ->
        resolution
        |> Absinthe.Resolution.put_result({:error, "unauthenticated"})
    end
  end
end

By specifying @behaviour Absinthe.Middleware the compiler will ensure that we provide a def call callback. This function takes an %Absinthe.Resolution{} struct and will also need to return one such struct.

On that struct there is a context key which holds the absinthe context. This is generally where things like the current user are placed. For more information on how the current user ends up in the context please see our full authentication guide on the website.

Our call/2 function simply checks the context to see if there is a current user. If there is, we pass the resolution onward. If there is not, we update the resolution state to :resolved and place an error result.

Middleware can be placed on a field in three different ways:

1. Using the Absinthe.Schema.Notation.middleware/2 macro used inside a field definition.
2. Using the middleware/3 callback in your schema.
3. Returning a {:middleware, middleware_spec, config} tuple from a resolution function.

The middleware/2 macro

For placing middleware on a particular field, it's handy to use the middleware/2 macro.

Middleware will be run in the order in which they are specified. The middleware/3 callback has final say on what middleware get set.

Examples

MyApp.Web.Authentication would run before resolution, and HandleError would run after.

field :hello, :string do
  middleware MyApp.Web.Authentication
  resolve &get_the_string/2
  middleware HandleError, :foo
end

Anonymous functions are a valid middleware spec. A nice use case is altering the context in a logout mutation. Mutations are the only time the context should be altered. This is not enforced.

field :logout, :query do
  middleware fn res, _ ->
    %{res |
      context: Map.delete(res.context, :current_user),
      value: "logged out",
      state: :resolved
    }
  end
end

middleware/2 even accepts local public function names. Note that middleware/2 is the only thing that can take local function names without an associated module. If not using macros, use {{__MODULE__, :function_name}, []}

def auth(res, _config) do
  # auth logic here
end

query do
  field :hello, :string do
    middleware :auth
    resolve &get_the_string/2
  end
end

The middleware/3 callback

middleware/3 is a function callback on a schema. When you use Absinthe.Schema a default implementation of this function is placed in your schema. It is passed the existing middleware for a field, the field itself, and the object that the field is a part of.

So for example if your schema contained:

object :user do
  field :name, :string
  field :age, :integer
end

query do
  field :lookup_user, :user do
    resolve fn _, _ ->
      {:ok, %{name: "Bob"}}
    end
  end
end

def middleware(middleware, field, object) do
  middleware |> IO.inspect
  field |> IO.inspect
  object |> IO.inspect

  middleware
end

Given a document like:

{ lookupUser { name }}

object is each object that is accessed while executing the document. In our case that is the :user object and the :query object. field is every field on that object, and middleware is a list of whatever middleware spec have been configured by the schema on that field. Concretely then, the function will be called , with the following arguments:

YourSchema.middleware([{Absinthe.Resolution, #Function<20.52032458/0>}], lookup_user_field_of_root_query_object, root_query_object)
YourSchema.middleware([{Absinthe.Middleware.MapGet, :name}], name_field_of_user, user_object)
YourSchema.middleware([{Absinthe.Middleware.MapGet, :age}], age_field_of_user, user_object)

In the latter two cases we see that the middleware list is empty. In the first case we see one middleware spec, which is placed by the resolve macro used in the :lookup_user field.

Default Middleware

One use of middleware/3 is setting the default middleware on a field. By default middleware is placed on a field that looks up a field by its snake case identifier, ie :resource_name. Here is an example of how to change the default to use a camel cased string, IE, "resourceName".

def middleware(middleware, %{identifier: identifier} = field, object) do
  camelized =
    identifier
    |> Atom.to_string
    |> Macro.camelize

  new_middleware_spec = {{__MODULE__, :get_camelized_key}, camelized}

  Absinthe.Schema.replace_default(middleware, new_middleware_spec, field, object)
end

def get_camelized_key(%{source: source} = res, key) do
  %{res | state: :resolved, value: Map.get(source, key)}
end

There's a lot going on here so let's unpack it. We need to define a specification to tell Absinthe what middleware to run. The form we're using is {{MODULE, :function_to_call}, options_of_middleware}. For our purposes we're simply going to use a function in the schema module itself get_camelized_key.

We then use the Absinthe.Schema.replace_default/4 function to swap out the default middleware already present in the middleware list with the new one we want to use. It handles going through the existing list of middleware and seeing if it's using the default or if it has custom resolvers on it. If it's using the default, the function applies our newly defined middleware spec.

Like all middleware functions, :get_camelized_key takes a resolution struct, and options. The options is the camelized key we generated. We get the camelized string from the parent map, and set it as the value of the resolution struct. Finally we mark the resolution state :resolved.

Side note: This middleware/3 function is called whenever we pull the type out of the schema. The middleware itself is run every time we get a field on an object. If we have 1000 objects and we were doing the camelization logic INSIDE the middleware, we would compute the camelized string 1000 times. By doing it in the def middleware callback we do it just once.

Changes Since 1.3

In Absinthe 1.3, fields without any middleware/2 or resolve/1 calls would show up with an empty list [] as its middleware in the middleware/3 function. If no middleware was applied in the function and it also returned [], THEN Absinthe would apply the default.

This made it very easy to accidentally break your schema if you weren't particularly careful with your pattern matching. Now the defaults are applied FIRST by absinthe, and THEN passed to middleware/3. Consequently, the middleware list argument should always have at least one value. This is also why there is now the replace_default/4 function, because it handles telling the difference between a field with a resolver and a field with the default.

Object Wide Authentication

Let's use our authentication middleware from earlier, and place it on every field in the query object.

defmodule MyApp.Web.Schema do
  use Absinthe.Schema

  query do
    field :private_field, :string do
      resolve fn _, _ ->
        {:ok, "this can only be viewed if authenticated"}
      end
    end
  end

  def middleware(middleware, _field, %Absinthe.Type.Object{identifier: identifier})
  when identifier in [:query, :subscription, :mutation] do
    [MyApp.Web.Authentication | middleware]
  end
  def middleware(middleware, _field, _object) do
    middleware
  end
end

It is important to note that we are matching for the :query, :subscription or :mutation identifier types. We do this because the middleware function will be called for each field in the schema. If we didn't limit it to those types, we would be applying authentication to every field in the entire schema, even stuff like :name or :age. This generally isn't necessary provided you authenticate at the entrypoints.

Main Points

• Middleware functions take a %Absinthe.Resolution{} struct, and return one.
• All middleware on a field are always run, make sure to pattern match on the state if you care.