Patterns in Ruby: Query Object

[joshmfrankel]

The Query Object Pattern

Description

• Return Something / Change Nothing
• Request a collection of data
• File name describes the Query result
• Always returns an ActiveRecord chain-able result (unless Enumerable)

When to use it

• You have a complicated ActiveRecord chain
• You have filters, sorting, pagination, or other configurable arguments
• You need to use same query in multiple locations
• You need to utilize raw SQL

When not to use it (Anti-patterns)

• You need to produce a side-effect
• You have a simple query that could be expressed as a Model scope

Before

class PostsController < ApplicationController
  def index
    filters = params[:filter]

    posts = current_user.organization.posts
    posts = posts.where(category: filters[:category]) if filters[:category].present?
    posts = posts.where(author: filters[:author]) if filters[:author].present?
    posts = posts.where(publised_at: filters[:publised_at_start]..filters[:publised_at_end]) if filters[:publised_at_start].present? && filters[:publised_at_end].present?
      
    posts = posts.order("#{params[:sort][:column]} #{params[:sort][:direction]}, created_at DESC") if params[:sort].present?

    @pagy, @posts = pagy(posts, page: params[:pagination][:page], per_page: params[:pagination][:per_page])
  end
end

After

class PostsController < ApplicationController
  def index
    @pagy, @posts = FilteredPostsQuery.new(
      scope: current_user.organization.posts, 
      filters: params[:filters],
      sort: params[:sort],
      pagination: params[:pagination]
    ).call
  end
end

class FilteredPostsQuery < ApplicationQuery
  attr_reader :scope, :filters, :sort, :pagination

  def initialize(scope:, filters:, sort:, pagination:)
    @scope = scope
    @filters = filters
    @sort = sort
    @pagination = pagination
  end

  def call
    result = filtered_posts(scope)
    result = sorted_posts(result)

    pagy(
      result,
      page: pagination[:page], 
      per_page: pagination[:per_page]
    )
  end

  private

  def filtered_posts(scope)
    scope = scope.where(category: filters[:category]) if filters[:category].present?
    scope = scope.where(author: filters[:author]) if filters[:author].present?
    scope = scope.where(publised_at: filters[:publised_at_start]..filters[:publised_at_end]) if filters[:publised_at_start].present? && filters[:publised_at_end].present?
    scope
  end

  def sorted_posts(result)
    result.order("#{sort[:column]} #{sort[:direction]}, created_at DESC") if sort.present?
    result
  end
end