# Output Fields

Flask-RESTful provides an easy way to control what data you actually render in your response. With the `fields` module, you can use whatever objects (ORM models/custom classes/etc.) you want in your resource. `fields` also lets you format and filter the response so you don’t have to worry about exposing internal data structures.

It’s also very clear when looking at your code what data will be rendered and how it will be formatted.

## Basic Usage

You can define a dict or OrderedDict of fields whose keys are names of attributes or keys on the object to render, and whose values are a class that will format & return the value for that field. This example has three fields: two are `String` and one is a `DateTime`, formatted as an RFC 822 date string (ISO 8601 is supported as well)

    from flask_restful import Resource, fields, marshal_with

    resource_fields = {
        'name': fields.String,
        'address': fields.String,
        'date_updated': fields.DateTime(dt_format='rfc822'),
    }

    class Todo(Resource):
        @marshal_with(resource_fields, envelope='resource')
        def get(self, **kwargs):
            return db_get_todo()  # Some function that queries the db

This example assumes that you have a custom database object (`todo`) that has attributes `name`, `address`, and `date_updated`. Any additional attributes on the object are considered private and won’t be rendered in the output. An optional `envelope` keyword argument is specified to wrap the resulting output.

The decorator `marshal_with` is what actually takes your data object and applies the field filtering. The marshalling can work on single objects, dicts, or lists of objects.

Note

`marshal_with` is a convenience decorator, that is functionally equivalent to

    class Todo(Resource):
        def get(self, **kwargs):
            return marshal(db_get_todo(), resource_fields), 200

This explicit expression can be used to return HTTP status codes other than 200 along with a successful response (see `abort()` for errors).

## Renaming Attributes

Often times your public facing field name is different from your internal field name. To configure this mapping, use the `attribute` keyword argument.

    fields = {
        'name': fields.String(attribute='private_name'),
        'address': fields.String,
    }

A lambda (or any callable) can also be specified as the `attribute`

    fields = {
        'name': fields.String(attribute=lambda x: x._private_name),
        'address': fields.String,
    }

Nested properties can also be accessed with `attribute`

    fields = {
        'name': fields.String(attribute='people_list.0.person_dictionary.name'),
        'address': fields.String,
    }

## Default Values

If for some reason your data object doesn’t have an attribute in your fields list, you can specify a default value to return instead of `None`.

    fields = {
        'name': fields.String(default='Anonymous User'),
        'address': fields.String,
    }

## Custom Fields & Multiple Values

Sometimes you have your own custom formatting needs. You can subclass the `fields.Raw` class and implement the `format` function. This is especially useful when an attribute stores multiple pieces of information. e.g. a bit-field whose individual bits represent distinct values. You can use fields to multiplex a single attribute to multiple output values.

This example assumes that bit 1 in the `flags` attribute signifies a “Normal” or “Urgent” item, and bit 2 signifies “Read” or “Unread”. These items might be easy to store in a bitfield, but for a human readable output it’s nice to convert them to separate string fields.

    class UrgentItem(fields.Raw):
        def format(self, value):
            return "Urgent" if value & 0x01 else "Normal"

    class UnreadItem(fields.Raw):
        def format(self, value):
            return "Unread" if value & 0x02 else "Read"

    fields = {
        'name': fields.String,
        'priority': UrgentItem(attribute='flags'),
        'status': UnreadItem(attribute='flags'),
    }

## Url & Other Concrete Fields

Flask-RESTful includes a special field, `fields.Url`, that synthesizes a uri for the resource that’s being requested. This is also a good example of how to add data to your response that’s not actually present on your data object.:

    class RandomNumber(fields.Raw):
        def output(self, key, obj):
            return random.random()

    fields = {
        'name': fields.String,
        # todo_resource is the endpoint name when you called api.add_resource()
        'uri': fields.Url('todo_resource'),
        'random': RandomNumber,
    }

By default `fields.Url` returns a relative uri. To generate an absolute uri that includes the scheme, hostname and port, pass the keyword argument `absolute=True` in the field declaration. To override the default scheme, pass the `scheme` keyword argument:

    fields = {
        'uri': fields.Url('todo_resource', absolute=True),
        'https_uri': fields.Url('todo_resource', absolute=True, scheme='https')
    }

## Complex Structures

You can have a flat structure that `marshal()` will transform to a nested structure

    >>> from flask_restful import fields, marshal
    >>> import json
    >>>
    >>> resource_fields = {'name': fields.String}
    >>> resource_fields['address'] = {}
    >>> resource_fields['address']['line 1'] = fields.String(attribute='addr1')
    >>> resource_fields['address']['line 2'] = fields.String(attribute='addr2')
    >>> resource_fields['address']['city'] = fields.String
    >>> resource_fields['address']['state'] = fields.String
    >>> resource_fields['address']['zip'] = fields.String
    >>> data = {'name': 'bob', 'addr1': '123 fake street', 'addr2': '', 'city': 'New York', 'state': 'NY', 'zip': '10468'}
    >>> json.dumps(marshal(data, resource_fields))
    '{"name": "bob", "address": {"line 1": "123 fake street", "line 2": "", "state": "NY", "zip": "10468", "city": "New York"}}'

Note

The address field doesn’t actually exist on the data object, but any of the sub-fields can access attributes directly from the object as if they were not nested.

## List Field

You can also unmarshal fields as lists

    >>> from flask_restful import fields, marshal
    >>> import json
    >>>
    >>> resource_fields = {'name': fields.String, 'first_names': fields.List(fields.String)}
    >>> data = {'name': 'Bougnazal', 'first_names' : ['Emile', 'Raoul']}
    >>> json.dumps(marshal(data, resource_fields))
    >>> '{"first_names": ["Emile", "Raoul"], "name": "Bougnazal"}'

## Advanced : Nested Field

While nesting fields using dicts can turn a flat data object into a nested response, you can use `Nested` to unmarshal nested data structures and render them appropriately.

    >>> from flask_restful import fields, marshal
    >>> import json
    >>>
    >>> address_fields = {}
    >>> address_fields['line 1'] = fields.String(attribute='addr1')
    >>> address_fields['line 2'] = fields.String(attribute='addr2')
    >>> address_fields['city'] = fields.String(attribute='city')
    >>> address_fields['state'] = fields.String(attribute='state')
    >>> address_fields['zip'] = fields.String(attribute='zip')
    >>>
    >>> resource_fields = {}
    >>> resource_fields['name'] = fields.String
    >>> resource_fields['billing_address'] = fields.Nested(address_fields)
    >>> resource_fields['shipping_address'] = fields.Nested(address_fields)
    >>> address1 = {'addr1': '123 fake street', 'city': 'New York', 'state': 'NY', 'zip': '10468'}
    >>> address2 = {'addr1': '555 nowhere', 'city': 'New York', 'state': 'NY', 'zip': '10468'}
    >>> data = { 'name': 'bob', 'billing_address': address1, 'shipping_address': address2}
    >>>
    >>> json.dumps(marshal_with(data, resource_fields))
    '{"billing_address": {"line 1": "123 fake street", "line 2": null, "state": "NY", "zip": "10468", "city": "New York"}, "name": "bob", "shipping_address": {"line 1": "555 nowhere", "line 2": null, "state": "NY", "zip": "10468", "city": "New York"}}'

This example uses two `Nested` fields. The `Nested` constructor takes a dict of fields to render as sub-fields. The important difference between the `Nested` constructor and nested dicts (previous example), is the context for attributes. In this example, `billing_address` is a complex object that has its own fields and the context passed to the nested field is the sub-object instead of the original `data` object. In other words: `data.billing_address.addr1` is in scope here, whereas in the previous example `data.addr1` was the location attribute. Remember: `Nested` and `List` objects create a new scope for attributes.

Use `Nested` with `List` to marshal lists of more complex objects:

    user_fields = {
        'id': fields.Integer,
        'name': fields.String,
    }

    user_list_fields = {
        fields.List(fields.Nested(user_fields)),
    }