how to retrieve data from the database using the has_many association - ruby-on-rails

Question

I have
message table
and
message_pictures
table(It belongs to the message table) and what I want to do is retrieve each messsge pictures (if there is any) from the
message_controller
.
How can I do it. I have surfed the web but found little scanty explanations on it.
This is the message class(model)
class Message < ApplicationRecord
belongs_to :user
has_many :message_pictures, dependent: :destroy
accepts_nested_attributes_for :message_pictures
end
This is the message_pictures class(model)
class MessagePicture < ApplicationRecord
belongs_to :message, optional: true
mount_uploader :message_pictures, PictureUploader
end
and this is the index method of the message_controller class
def index
#user = User.find(current_user.id)#414, 449, 494
#messages = #user.messages.paginate(page: params[:page])
##messages = #messages.message_pictures.paginate(page: params[:page])
end
You can see the line 4 of the index method to see the way I did mine but its not working

Answer 1

I believe what you need is has_many ... :through
app/models/user.rb
class User < ApplicationRecord
# ...
has_many :messages, dependent: :destroy
has_many :message_pictures, through: :messages
end
app/controllers/messages_controller.rb
class MessagesController < ApplicationController
# ...
def index
#user = User.find(current_user.id)
#messages = #user.messages.paginate(page: params[:page])
#message_pictures = #user.message_pictures
end
end
has_many ... :through simplifies the retrieving of "nested" children records via "SQL JOINS", of which normally you would have done it in a longer (more explicit way) like the following (which also works):
class MessagesController < ApplicationController
# ...
def index
#user = User.find(current_user.id)
#messages = #user.messages.paginate(page: params[:page])
#message_pictures = MessagePicture.joins(message: :user).where(
messages: { # <-- this needs to be the table name, and not the association name, and is why it is in plural form
users: { # <-- this needs to be the table name, and not the association name, and is why it is in plural form
id: #user.id
}
}
)
end
end
Update: Alternative Solution
Looking back at your question, I have a feeling you'd only want #message_pictures that corresponds to #messages and not to all#user.messages, because I noticed you have pagination for the messages. I'll do it like this instead:
app/controllers/messages_controller.rb
class MessagesController < ApplicationController
# ...
def index
#user = User.find(current_user.id)
# the `includes` here prevents N+1 SQL queries, because we are gonna loop
# through each `message_picture` in each `message` record (see index.html.erb below)
#messages = #user.messages.includes(:message_pictures).paginate(page: params[:page])
end
end
app/views/messages/index.html.erb (example)
<h1>Messages:</h1>
<% #messages.each do |message| %>
<section>
<h2>Message:</h2>
<p><%= message.content %></p>
<h3>Message Pictures:<h3>
<div>
<% message.message_pictures.each do |message_picture| %>
<% message_picture.message_pictures.each do |message_picture_attachment| %>
<%= image_tag message_picture_attachment.url.to_s %>
<% end %>
<br>
<% end %>
</div>
</section>
<% end %>
^ Above assumes MessagePicture is using carrierwave. P.S. It looks to me there's something wrong with how you defined your models, because your message has many message_pictures, while each of the message_picture also has many attached message_picture carrierwave attachments (assuming you used the "multiple-file" upload set up for carrierwave because you used mount_uploader :message_pictures, PictureUploader instead of mount_uploader :message_picture, PictureUploader. The model problem I think is because it's like this: message < message_pictures < message_pictures attachments, but (depending on your use-case), it should probably be just like message < message_pictures - message_picture attachment, or just simply message < message_pictures attachments