Inquery

A skeleton that allows extracting queries into atomic, reusable classes.

Installation

To install the Inquery gem:

$ gem install inquery

To install it using bundler (recommended for any application), add it to your Gemfile:

gem 'inquery'

Basic usage

class FetchUsersWithACar < Inquery::Query
  schema(
    color: :symbol
  )

  def call
    User.joins(:cars).where(cars: { color: osparams.color })
  end
end

FetchUsersWithACar.run
# => [<User id: 1 ...]

Inquery offers its functionality trough two query base classes: Inquery::Query and Inquery::Query::Chainable. See the following sections for detailed explanations.

Basic queries

Basic queries inherit from Inquery::Query. They receive an optional set of parameters and commonly return a relation / AR result. An optional process method lets you perform additional result processing steps if needed (i.e. converting the result to a hash or similar).

For this basic functionality, inherit from Inquery::Query and overwrite the call and optionally the process method:

class FetchRedCarsAsJson
  # The `call` method must be overwritten for every query. It is usually called
  # via `run`.
  def call
    Car.where(color: 'red')
  end

  # The `process` method can optionally be overwritten. The base implementation
  # just returns the unprocessed `results` argument.
  def process(results)
    results.to_json
  end
end

Queries can be called in various ways:

# Instantiates the query class and runs `call` and `process`.
FetchRedCarsAsJson.run(params = {})

# Instantiates the query class and runs `call`. No result processing
# is done.
FetchRedCarsAsJson.call(params = {})

# You can also instantiate the query class manually.
FetchRedCarsAsJson.new(params = {}).run

# Or just run the `call` method without `process`.
FetchRedCarsAsJson.new(params = {}).call

Note that it's perfectly fine for some queries to return nil, i.e. if they're writing queries that don't fetch any results.

Chainable queries

Chainable queries are queries that input and output an Active Record relation. You can access the given relation using the method relation:

class Queries::User::FetchActive < Inquery::Query::Chainable
  def call
    relation.where(active: 1)
  end
end

Input and output relations may or may not be of the same AR class (i.e. you could pass a relation of Groups and receive back a relation of corresponding Users).

Relation validation

Chainable queries allow you to further specify and validate the relation it receives. This is done using the static relation method:

class Queries::User::FetchActive < Inquery::Query::Chainable
  # This will raise an exception when passing a relation which does not
  # correspond to the `User` model.
  relation class: 'User'

  # ....
end

The relation method accepts the following options:

class

Allows to restrict the class (attribute klass) of the relation. Use nil to not perform any checks. The class attribute will also be taken to infer a default if no relation is given and you didn't specify any default.
default

This allows to specify a default relation that will be taken if no relation is given. This must be specified as a Proc returning the relation. Set this to false for no default. If this is set to nil, it will try to infer the default from the option class (if given).
fields

Allows to restrict the number of fields / values the relation must select. This is particularly useful if you're using the query as a subquery and need it to return exactly one field. Use nil to not perform any checks.
default_select

If this is set to a symbol, the relation does not have any select fields specified (select_values is empty) and fields is > 0, it will automatically select the given field. This option defaults to :id. Use nil to disable this behavior.

Using query classes as regular scopes

Chainable queries can also be used as regular AR model scopes:

class User < ActiveRecord::Base
  scope :active, Queries::User::FetchActive
end

class Queries::User::FetchActive < Inquery::Query::Chainable
  # Note that specifying either `class` or `default` is mandatory when using
  # this query class as a scope. The reason for this is that, if the scope is
  # otherwise empty, the class will receive `nil` from AR and therefore has no
  # way of knowing which default class to take.
  relation class: 'User'

  def call
    relation.where(active: 1)
  end
end

This approach allows to you use short and descriptive code like User.active but have the possibly complex query code hidden in a separate, reusable class.

Note that when using classes as scopes, the process method will be ignored.

Using the given relation as subquery

In simple cases and all the examples above, we just extend the given relation and return it again. It is also possible however to just use the given relation as a subquery and return a completely new relation:

class FetchUsersInGroup < Inquery::Query::Chainable
  # Here we do not specify any specific class, as we don't care for it as long
  # as the relation returns exactly one field.
  relation fields: 1

  def call
    return ::User.where(%(
      id IN (
        SELECT user_id FROM GROUPS_USERS WHERE group_id IN (
          #{relation.to_sql}
        )
      )
    ))
  end
end

This query could then be called in the following ways:

FetchUsersInGroup.run(
  GroupsUser.where(user_id: 1).select(:group_id)
)

# In this example, we're not specifying any select for the relation we pass to
# the query class. This is fine because the query automatically defaults to
# selecting `id` if exactly one field is required (`fields: 1`) and no select is
# specifyed. You can control this further with the option `default_select`.
FetchUsersInGroup.run(Group.where(color: 'red'))

Parameters

Both query classes can be parameterized using a hash called params. It is recommended to specify and validate input parameters in every query. For this purpose, Inquery provides the schema method witch integrates the Schemacop validation Gem:

class SomeQueryClass < Inquery::Query
  schema(
    some_param: :integer,
    some_other_param: {
      hash: {
        some_field: :string
      }
    }
  )

  # ...
end

The schema is validated at query class instantiation. An exception will be raised if the given params do not match the schema specified. See documentation of the Schemacop Gem for more information on how to specify schemas.

Parameters can be accessed using either params or osparams. The method osparams automatically wraps params in an OpenStruct for more convenient access.

class SomeQueryClass < Inquery::Query
  def run
    User.where(
      active: params[:active],
      username: osparams.search
    )
  end
end

Rails integration

While it is optional, Inquery has been written from the ground up to be perfectly integrated into any Rails application. It has proven to be a winning concept to extract all complex queries into separate classes that are independently executable and testable.

Directory structure

While not enforced, it is encouraged to use the following structure for storing your query classes:

All domain-specific query classes reside in app/queries.
They're in the module Queries.
Queries are further grouped by the model they return (and not the model they receive). For instance, a class fetching all active users could be located at Queries::User::FetchActive and would reside under app/queries/user/fetch_active.rb.

There are some key benefits to this approach:

As it should, domain-specific code is located within app/.
As queries are grouped by the model they return and consistently named, they're easy to locate and it does not take much thought where to put and how to name new query classes.
As there is a single file per query class, it's a breeze to list all queries, i.e. to check their naming for consistency.
If you're using the same layout for your unit tests, it is absolutely clear where to find the corresponding unit tests for each one of your query classes.