Grouping Query with Has Many Relations in Rails - sql

Question

I have a has_many, through: relationship between Projets and Categories using a Categorization model. A Projet belongs_to a Client.
class Client < ApplicationRecord
has_many :projets
end
class Category < ApplicationRecord
has_many :categorizations, dependent: :destroy
has_many :projets, through: :categorizations
end
class Categorization < ApplicationRecord
belongs_to :category
belongs_to :projet
end
class Projet < ApplicationRecord
belongs_to :client
has_many :categorizations, dependent: :destroy
has_many :categories, through: :categorizations
end
For a specific category I'd like to list all the projets, grouped by the client. e.g.
(for category_id = 3)
Client A
Projet 1
Projet 2
Client B
Projet 3
Client C
Projet 4
So far I can get this working, but only by using two queries (one of which is very inefficient (n+1 problem).
This is the code
def listing
#projets_clients = Projet
.select("client_id")
.includes(:client)
.joins(:categorizations)
.where(categorizations: { category: #category })
.group("client_id")
#clients = []
#projets_clients.each do |p|
#clients << Client.includes(:projets).find(p.client_id)
end
end
If anyone can suggest a better approach I'd love to learn how to optimise this as I haven't been able to find a better way myself.
Thanks.

Answer 1

There are a few different ways to do this. For complex queries I sometimes find it easier to write and execute straight SQL. However for your case, depending on data size, you could just eager load the data and turn it into a hash.
Note: When I was testing this code I used projects instead of projets.
#category = Category.includes(projects: [:client]).find(2)
#projects_by_client = #category.projects.group_by(&:client_id)
# In your view
<%- #projects_by_client.each do |client_id, projects| %>
<%= projects.first.client.name %>
<%- projects.each do |project| %>
<%= project.name %>
<% end %>
<% end %>
A more fleshed out solution might use full sql with a query object and presenter object. I spun up a quick project using the below code and the output is what you are looking for.
# app/controllers/clients_controller.rb
class ClientsController < ApplicationController
def show
result = ClientQuery.call(params[:id])
#presenter = ClientPresenter.new(result)
end
end
# app/services/client_query.rb
class ClientQuery
class << self
def call(client_id)
sql_query(client_id)
end
protected
def sql_query(client_id)
ActiveRecord::Base.
connection.
execute(
sanitized_sql_statement(client_id)
)
end
def sanitized_sql_statement(client_id)
ActiveRecord::Base.send(
:sanitize_sql_array,
[
sql_statement,
client_id
]
)
end
def sql_statement
<<-SQL
SELECT
c.id AS client_id,
c.name AS client_name,
p.name AS project_name
FROM
clients c
INNER JOIN
projects p ON p.client_id = c.id
INNER JOIN
categorizations cz ON cz.project_id = p.id
INNER JOIN
categories ct ON ct.id = cz.category_id
WHERE
ct.id = ?;
SQL
end
end
end
# app/presenters/client_presenter.rb
class ClientPresenter
attr_reader :clients
def initialize(data)
#clients = {}
process_sql_result(data)
end
private
def process_sql_result(data)
data.each do |row|
client_id = row['client_id']
#clients[client_id] ||= { client_name: row['client_name'] }
#clients[client_id][:projects] ||= []
#clients[client_id][:projects] << row['project_name']
end
end
end
# app/views/show.html.erb
<%- #presenter.clients.each do |client_id, client_presenter| %>
<h1><%= client_presenter[:client_name] %></h1>
<ul>
<%- client_presenter[:projects].each do |project_name| %>
<li><%= project_name %></li>
<% end %>
</ul>
<% end %>
This is of course just one of many ways you could go about getting your data in a single query and presenting it.