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103 lines
3.2 KiB
ReStructuredText
103 lines
3.2 KiB
ReStructuredText
Nodes
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=====
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A ``Node`` is an Interface provided by ``graphene.relay`` that contains
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a single field ``id`` (which is a ``ID!``). Any object that inherits
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from it has to implement a ``get_node`` method for retrieving a
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``Node`` by an *id*.
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Quick example
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-------------
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Example usage (taken from the `Starwars Relay example`_):
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.. code:: python
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class Ship(graphene.ObjectType):
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'''A ship in the Star Wars saga'''
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class Meta:
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interfaces = (relay.Node, )
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name = graphene.String(description='The name of the ship.')
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@classmethod
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def get_node(cls, info, id):
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return get_ship(id)
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The ``id`` returned by the ``Ship`` type when you query it will be a
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scalar which contains enough info for the server to know its type and
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its id.
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For example, the instance ``Ship(id=1)`` will return ``U2hpcDox`` as the
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id when you query it (which is the base64 encoding of ``Ship:1``), and
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which could be useful later if we want to query a node by its id.
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Custom Nodes
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------------
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You can use the predefined ``relay.Node`` or you can subclass it, defining
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custom ways of how a node id is encoded (using the ``to_global_id`` method in the class)
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or how we can retrieve a Node given a encoded id (with the ``get_node_from_global_id`` method).
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Example of a custom node:
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.. code:: python
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class CustomNode(Node):
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class Meta:
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name = 'Node'
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@staticmethod
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def to_global_id(type_, id):
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return f"{type_}:{id}"
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@staticmethod
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def get_node_from_global_id(info, global_id, only_type=None):
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type_, id = global_id.split(':')
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if only_type:
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# We assure that the node type that we want to retrieve
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# is the same that was indicated in the field type
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assert type_ == only_type._meta.name, 'Received not compatible node.'
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if type_ == 'User':
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return get_user(id)
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elif type_ == 'Photo':
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return get_photo(id)
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The ``get_node_from_global_id`` method will be called when ``CustomNode.Field`` is resolved.
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Accessing node types
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--------------------
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If we want to retrieve node instances from a ``global_id`` (scalar that identifies an instance by it's type name and id),
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we can simply do ``Node.get_node_from_global_id(info, global_id)``.
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In the case we want to restrict the instance retrieval to a specific type, we can do:
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``Node.get_node_from_global_id(info, global_id, only_type=Ship)``. This will raise an error
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if the ``global_id`` doesn't correspond to a Ship type.
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Node Root field
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---------------
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As is required in the `Relay specification`_, the server must implement
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a root field called ``node`` that returns a ``Node`` Interface.
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For this reason, ``graphene`` provides the field ``relay.Node.Field``,
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which links to any type in the Schema which implements ``Node``.
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Example usage:
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.. code:: python
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class Query(graphene.ObjectType):
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# Should be CustomNode.Field() if we want to use our custom Node
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node = relay.Node.Field()
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.. _Relay specification: https://facebook.github.io/relay/docs/graphql-relay-specification.html
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.. _Starwars Relay example: https://github.com/graphql-python/graphene/blob/master/examples/starwars_relay/schema.py
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