introduction.md

layout	doc_stub	search	section	title	desc	index
guide	false	true	Fields	Introduction	Implement fields and resolvers with the Ruby DSL	0

Object fields expose data about that object or connect the object to other objects. You can add fields to your object types with the field(...) class method, for example:

field :name, String, "The unique name of this list", null: false

{% internal_link "Objects", "/type_definitions/objects" %} and {% internal_link "Interfaces", "/type_definitions/interfaces" %} have fields.

The different elements of field definition are addressed below:

• Return types say what kind of data this field returns
• Documentation includes description and deprecation notes
• Resolution behavior hooks up Ruby code to the GraphQL field
• Arguments allow fields to take input when they're queried
• Extra field metadata for low-level access to the GraphQL-Ruby runtime
• Add default values for field parameters

Field Return Type

The second argument to field(...) is the return type. This can be:

• A built-in GraphQL type (Integer, Float, String, ID, or Boolean)
• A GraphQL type from your application
• An array of any of the above, which denotes a {% internal_link "list type", "/type_definitions/lists" %}.

{% internal_link "Nullability", "/type_definitions/non_nulls" %} is expressed with the null: keyword:

• null: true (default) means that the field may return nil
• null: false means the field is non-nullable; it may not return nil. If the implementation returns nil, GraphQL-Ruby will return an error to the client.

Additionally, list types maybe nullable by adding [..., null: true] to the definition.

Here are some examples:

field :name, String # `String`, may return a `String` or `nil`
field :id, ID, null: false # `ID!`, always returns an `ID`, never `nil`
field :teammates, [Types::User], null: false # `[User!]!`, always returns a list containing `User`s
field :scores, [Integer, null: true] # `[Int]`, may return a list or `nil`, the list may contain a mix of `Integer`s and `nil`s

Field Documentation

Fields may be documented with a description and may be deprecated.

Descriptions can be added with the field(...) method as a positional argument, a keyword argument, or inside the block:

# 3rd positional argument
field :name, String, "The name of this thing", null: false

# `description:` keyword
field :name, String, null: false,
  description: "The name of this thing"

# inside the block
field :name, String, null: false do
  description "The name of this thing"
end

Deprecated fields can be marked by adding a deprecation_reason: keyword argument:

field :email, String,
  deprecation_reason: "Users may have multiple emails, use `User.emails` instead."

Fields with a deprecation_reason: will appear as "deprecated" in GraphiQL.

Field Resolution

In general, fields return Ruby values corresponding to their GraphQL return types. For example, a field with the return type String should return a Ruby string, and a field with the return type [User!]! should return a Ruby array with zero or more User objects in it.

By default, fields return values by:

• Trying to call a method on the underlying object; OR
• If the underlying object is a Hash, lookup a key in that hash.
• An optional :fallback_value can be supplied that will be used if the above fail.

The method name or hash key corresponds to the field name, so in this example:

field :top_score, Integer, null: false

The default behavior is to look for a #top_score method, or lookup a Hash key, :top_score (symbol) or "top_score" (string).

You can override the method name with the method: keyword, or override the hash key(s) with the hash_key: or dig: keyword, for example:

# Use the `#best_score` method to resolve this field
field :top_score, Integer, null: false,
  method: :best_score

# Lookup `hash["allPlayers"]` to resolve this field
field :players, [User], null: false,
  hash_key: "allPlayers"

# Use the `#dig` method on the hash with `:nested` and `:movies` keys
field :movies, [Movie], null: false,
  dig: [:nested, :movies]

To pass-through the underlying object without calling a method on it, you can use method: :itself:

field :player, User, null: false,
  method: :itself

This is equivalent to:

field :player, User, null: false

def player
  object
end

If you don't want to delegate to the underlying object, you can define a method for each field:

# Use the custom method below to resolve this field
field :total_games_played, Integer, null: false

def total_games_played
  object.games.count
end

Inside the method, you can access some helper methods:

• object is the underlying application object (formerly obj to resolve functions)
• context is the query context (passed as context: when executing queries, formerly ctx to resolve functions)

Additionally, when you define arguments (see below), they're passed to the method definition, for example:

# Call the custom method with incoming arguments
field :current_winning_streak, Integer, null: false do
  argument :include_ties, Boolean, required: false, default_value: false
end

def current_winning_streak(include_ties:)
  # Business logic goes here
end

As the examples above show, by default the custom method name must match the field name. If you want to use a different custom method, the resolver_method option is available:

# Use the custom method with a non-default name below to resolve this field
field :total_games_played, Integer, null: false, resolver_method: :games_played

def games_played
  object.games.count
end

resolver_method has two main use cases:

1. resolver re-use between multiple fields
2. dealing with method conflicts (specifically if you have fields named context or object)

Note that resolver_method cannot be used in combination with method or hash_key.

Field Arguments

Arguments allow fields to take input to their resolution. For example:

• A search() field may take a term: argument, which is the query to use for searching, eg search(term: "GraphQL")
• A user() field may take an id: argument, which specifies which user to find, eg user(id: 1)
• An attachments() field may take a type: argument, which filters the result by file type, eg attachments(type: PHOTO)

Read more in the {% internal_link "Arguments guide", "/fields/arguments" %}

Extra Field Metadata

Inside a field method, you can access some low-level objects from the GraphQL-Ruby runtime. Be warned, these APIs are subject to change, so check the changelog when updating.

A few extras are available:

• ast_node
• graphql_name (the field's name)
• owner (the type that this field belongs to)
• lookahead (see {% internal_link "Lookahead", "/queries/lookahead" %})
• execution_errors, whose #add(err_or_msg) method should be used for adding errors
• argument_details (Intepreter only), an instance of {{ "GraphQL::Execution::Interpreter::Arguments" | api_doc }} with argument metadata
• parent (the previous object in the query)
• Custom extras, see below

To inject them into your field method, first, add the extras: option to the field definition:

field :my_field, String, null: false, extras: [:ast_node]

Then add ast_node: keyword to the method signature:

def my_field(ast_node:)
  # ...
end

At runtime, the requested runtime object will be passed to the field.

Custom extras are also possible. Any method on your field class can be passed to extras: [...], and the value will be injected into the method. For example, extras: [:owner] will inject the object type who owns the field. Any new methods on your custom field class may be used, too.

Field Parameter Default Values

The field method requires you to pass null: keyword argument to determine whether the field is nullable or not. For another field you may want to override camelize, which is true by default. You can override this behavior by adding a custom field with overwritten camelize option, which is true by default.

class CustomField < GraphQL::Schema::Field
  # Add `null: false` and `camelize: false` which provide default values
  # in case the caller doesn't pass anything for those arguments.
  # **kwargs is a catch-all that will get everything else
  def initialize(*args, null: false, camelize: false, **kwargs, &block)
    # Then, call super _without_ any args, where Ruby will take
    # _all_ the args originally passed to this method and pass it to the super method.
    super
  end
end

To use CustomField in your Objects and Interfaces, you'll need to register it as a field_class on those classes. See Customizing Fields for more information on how to do so.