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FastAPI Gem Version Build Status

Easily create robust, standardized API endpoints using lightning-fast database queries

FastAPI is a Rails library for querying interdependent datasets quickly and returning a human-readable, standard API output.

It works by constructing complex SQL queries that make efficient use of JOINs and subqueries based upon model dependencies (namely belongs_to, has_one, and has_many).

In only a few lines of code you can decide which fields you wish to expose to your endpoint, any filters you wish to run the data through, and create your controller.


You can preview a live example of FastAPI at

The repository is located at thestorefront/fastapi_example


This gem requires Oj >= 2.9.9 for JSONification, ActiveRecord >= 3.2.0, and ActiveSupport >= 3.2.0.

FastAPI currently supports PostegreSQL as a data layer.


FastAPI is available via RubyGems using:

$ gem install fastapi

Otherwise, in any Gemfile in a rails project, use:

require 'fastapi'


Let's say we have three models. Person, Bucket, and Marble. Each Bucket belongs to a Person and can have many Marbles.

Your model for Bucket might look something like this:

class Bucket < ActiveRecord::Base

  belongs_to :person
  has_many :marbles


Assume Bucket also has the fields :color, and :material.

Each Marble can have :color and :radius.

Every Person has a :name, :gender and :age.

We want to expose a list of Buckets as a JSONified API endpoint that contains records that look like the following:

    'id': 1,
    'color': 'blue',
    'material': 'plastic',
    'person': {
      'id': 107,
      'name': 'Mary-anne',
      'gender': 'Female',
      'age': 27
    'marbles': [
        'id': 22,
        'color': 'red',
        'radius': 5
        'id': 76,
        'color': 'green',
        'radius': 7

In order to do that we first look at our Bucket model and add the following:

class Bucket < ActiveRecord::Base

  belongs_to :person
  has_many :marbles

  # A "standard interface" is a list of user-exposed fields for the endpoint
  fastapi_standard_interface [


We then modify our Person model:

class Person < ActiveRecord::Base

  # Person is not top-level in the case of the "buckets"
  #   endpoint... we use a special setting indicating
  #   which fields to use if Person happens to be nested.

  # You can NOT include dependent fields here. (belongs_to, has_many)
  #   This is a hard-and-fast FastAPI rule that prevents overly
  #   complex nesting scenarios.

  fastapi_standard_interface_nested [


Keep in mind that this will only affect the cases where Person is a nested object.

If we wanted to expose a top-level Person API endpoint, we would use fastapi_standard_interface as well.

Finally, we must modify our Marble model in the same way:

class Marble < ActiveRecord::Base

  fastapi_standard_interface_nested [


Hmm... let's say we only want to list the Marbles that have a radius less than or equal to (<=) 10. Easy! We go back and modify our Bucket model. Add the following to class Bucket < ActiveRecord::Base:

  # top level filters affect the data that is shown,
  #   while filters on "has_many" fields affect which rows are shown per
  #   record
    marbles: {
      radius__lte: 10

Phew! We're almost done. Now to create the endpoint.

First open config/routes.rb and add the following:

namespace :api do
  namespace :v1, defaults: {format: :json} do
    resource :buckets

We now create a route to an API controller for Bucket in app/controllers/api/v1/buckets_controller.rb

(Can also use rails generate controller Api::V1::Buckets in the terminal):

class Api::V1::BucketsController < ApplicationController

  def index

    filters = request.query_parameters

    render json: Bucket.fastapi.filter(filters).response



Boom! Run your server with rails server and hop your way over to http://yourserver[:port]/api/v1/buckets to see your beautiful list of Buckets in the FastAPI standard JSON format. :)

Try to filter your datasets as well:

http://yourserver[:port]/api/v1/buckets/?color=red or


There are many to play with, go nuts!


FastAPI has four core components:

  1. ActiveRecord::Base extension that adds necessary class and instance methods.
  2. class FastAPI which is instantiated by an ActiveRecord::Base instance.
  3. Filters, which provide a way of easily interfacing with your data.
  4. FastAPI standard output, a strict way of displaying all FastAPI responses.

ActiveRecord::Base (Extension)



fastapi_standard_interface( fields [Array] )

Sets the standard interface for the top level of a fastapi response. Can use any available fields for the model, or belongs_to and has_many associations. Be sure to use the correct word form (singular vs. plural).


fastapi_standard_interface_nested( fields [Array] )

Sets the standard interface for the second level of a fastapi response (nested). Will be referred to whenever this model is found nested in another API response. Can use any available fields for the model, does not support associations.


fastapi_default_filters( filters [Hash] )

Sets any default filters for the top level fastapi response. Will be overridden if the same filter keys are provided when calling .filter on a FastAPI instance. See Filters section for more information on available filters.


fastapi_safe_fields( fields [Array] )

Sets safe fields for FastAPIInstance.safe_filter. These safe fields are a whitelist for filters, meaning safe_filter will only allow filtering by these fields.



Shorthand for the FastAPI constructor. Equivalent to Recommended usage is MyModel.fastapi.

class FastAPI

FastAPI instances provide a way to interface with your datasets and obtain necessary information (for an API response or otherwise).



initialize( model [Model < ActiveRecord::Base] )

Constructor. Automatically called using Model.fastapi, but can be used as Binds the provided Model to the FastAPI instance.


filter( filters [Hash] = {} , meta [Hash] = {} )

Compiles and executes an SQL query based on the supplied filters (see Filters section for more details). Can add additional fields to the expected meta response in the output, as keys in the meta Hash.


safe_filter( filters [Hash] = {} , meta [Hash] = {} )

Compiles and executes an SQL query based on the supplied filters (see Filters section for more details). Will only allow filtering by fields set in fastapi_safe_fields, or fastapi_standard_interface if not set. Can add additional fields to the expected meta response in the output, as keys in the meta Hash. Intended for use with filters = request.query_parameters.


fetch( id [Integer] , meta [Hash] = {} )

Similar to filter, but will retrieve a single object based on a single id. Ideal for show on a resource, as FastAPI will still format the response appropriately (and give a customized error for id not found).



Returns a Hash containing the data from the most recently executed filter or fetch call.



Returns a JSONified string containing the information in data



Returns a Hash containing the metadata from the most recently executed filter or fetch call.



Returns a JSONified string containing the information in meta



Returns a Hash containing both the data and metadata from the most recently executed filter or fetch call.



Intended to return the final API response. Returns a JSONified string containing the information available in the to_hash method.


reject( message [String] = 'Access Denied' )

Returns a JSONified string representing a standardized empty API response, with a provided error message. For example, if a user is not allowed to access a resource, you would call render json: Model.fastapi.reject.


Filters are a powerful tool in FastAPI that allow for granular control of your API responses. FastAPIInstance.filter accepts them, and they are also used in ActiveRecord::Base::fastapi_default_filters.

Filters work in the following way:

  key1: 2,
  key2: 'three'

Will grab a subset of all Models where :key1 is 2 and :key2 is 'three'.

Filter Comparators

What if we want to find a subset of Models where :key1 is greater than or equal to (>=) 5?

  key1__gte: 5

It's that easy. The double underscore indicates you're using a filter comparator, and gte stands for greater than or equal to.

The available comparators are as follows: (Descriptions marked with * indicate scalar inputs will be converted to arrays)

Scalar Fields

'is'              # Field == Value
'not'             # Field != Value
'gt'              # Field > Value
'gte'             # Field >= Value
'lt'              # Field < Value
'lte'             # Field <= Value
'like'            # Field contains Value (string)
'not_like'        # Field does not contain Value (string)
'ilike'           # Field contains Value (case ins.)
'not_ilike'       # Field does not contain Value (case ins.)
'null'            # Field is NULL
'not_null'        # Field is not NULL
'in'              # * Field is in Value
'not_in'          # * Field is not in Value

Array Fields

'subset'          # * Field is a subset of Value
'not_subset'      # * Field is not a subset of Value
'contains'        # * Value is a subset of Field
'not_contains'    # * Value is not a subset of Field
'intersects'      # * Field and Value have shared elements
'not_intersects'  # * Field and Value have no shared elements

If your key contains a double underscore, make sure to use the __is comparator if you look for a specific value.

Filters in HTTP Requests

If you'd like to allow for client-side data filtration (highly recommended), simply use the following in your API endpoint controller:

filters = request.query_parameters
render json: Model.fastapi.filter(filters).response

This will allow you to use filters (and their comparators) in the HTTP query parameters.

For example, http://yourapp/api/v1/users/?active=t&age__gte=19&age__gte=35 could return all active users between 19 and 35 years old.

Data Types in HTTP Requests

While using FastAPI, boolean fields are automatically detected, and the strings 't' and 'f' are converted to true and false, respectively. The same goes for integers. (Converted from string to int.)


In FastAPI, sorting is accomplished using a special filter: :__order

:__order Can be in the format of 'key', 'key,DIRECTION' or [:key, 'DIRECTION'] where DIRECTION is ASC or DESC. (Default ASC.)

An example, order users by age (ascending):

render json: User.fastapi.filter({__order: [:age, ASC]}).response

Or perhaps via HTTP (hitting an endpoint with request.query_parameters as the filter):



In FastAPI we opted for very robust, granular control of API responses. "Pages" do not exist in a strict sense, but rather by :__offset and :__count, much like you'd expect in a traditional database query.

For example,

render json: Model.fastapi.filter({__offset: 100, __count: 100}).response

Would return (up to) 100 results from Model, beginning at result number 100. (Page 2 at 100 results per page.)

Standard Output

FastAPI has a very strict, standard way of outputting data in the form of a response.

Responses will always look like the following:

  'meta': {
    'total': 0,
    'count': 0,
    'offset': 0,
    'error': null,
  'data': []

Where is the total number of records in the entire dataset, meta.count is the number of records in the response, meta.offset is the offset of the first record of the response, and meta.error is null if there was no error, or a string containing an error message if there was an error.

data will always be an Array of Objects. If there was an error with the response, data will be empty. If the response was formed by a FastAPIInstance.fetch call and a record was retrieved, data will be a length-1 Array.


Thanks for reading! We welcome contributors with good ideas, and we're always looking for new talent.

FastAPI was created by Keith Horwood and Trevor Strieber of Storefront, Inc. in 2014 and is (happily!) MIT licensed.

Twitter: @keithwhor, @TrevorStrieber, @Storefront

Github: keithwhor, TrevorS, thestorefront

Most recent version of the gem is available at fastapi


Easily create robust, standardized API endpoints using lightning-fast database queries







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