I have a need to do one query on a record set and get list of many type objects.
In this example I will use a blog post which a blog post has many different types.
Base Post:
class Post < ActiveRecord::Base
belongs_to :postable, :polymorphic => true
attr_accessible :body, :title
end
Audio Post:
class AudioPost < ActiveRecord::Base
attr_accessible :sound
has_one :postable, :as => :postable
end
Graphic Post:
class GraphicPost < ActiveRecord::Base
attr_accessible :image
has_one :postable, :as => :postable
end
This will allow me to do something like this.
#post = Post.all
#post.each do |post|
post.title
post.body
post.postable.image if post.postable_type == "GraphicPost"
post.postable.sound if post.postable_type == "AudioPost"
end
Though this works, it feels wrong to check the type because that goes against the duck type principle. I would assume there is a better way then this to do the same thing.
What is be a better design to achieve this same goal or am I just over thinking my design?
See my comments.
Anyway, if you want polymorphic, I would write logic in model:
class Post
delegate :content, to: :postable
class AudioPost
alias_method :sound, :content
class GraphicPost
alias_method :image, :content
You will want to render images different than a sound, for that part, I would use a helper:
module MediaHelper
def medium(data)
case # make your case detecting data type
# you could print data.class to see if you can discriminate with that.
and call in view
= medium post.content
Related
I read this interesting article about Using Polymorphism to Make a Better Activity Feed in Rails.
We end up with something like
class Activity < ActiveRecord::Base
belongs_to :subject, polymorphic: true
end
Now, if two of those subjects are for example:
class Event < ActiveRecord::Base
has_many :guests
after_create :create_activities
has_one :activity, as: :subject, dependent: :destroy
end
class Image < ActiveRecord::Base
has_many :tags
after_create :create_activities
has_one :activity, as: :subject, dependent: :destroy
end
With create_activities defined as
def create_activities
Activity.create(subject: self)
end
And with guests and tags defined as:
class Guest < ActiveRecord::Base
belongs_to :event
end
class Tag < ActiveRecord::Base
belongs_to :image
end
If we query the last 20 activities logged, we can do:
Activity.order(created_at: :desc).limit(20)
We have a first N+1 query issue that we can solve with:
Activity.includes(:subject).order(created_at: :desc).limit(20)
But then, when we call guests or tags, we have another N+1 query problem.
What's the proper way to solve that in order to be able to use pagination ?
Edit 2: I'm now using rails 4.2 and eager loading polymorphism is now a feature :)
Edit: This seemed to work in the console, but for some reason, my suggestion of use with the partials below still generates N+1 Query Stack warnings with the bullet gem. I need to investigate...
Ok, I found the solution ([edit] or did I ?), but it assumes that you know all subjects types.
class Activity < ActiveRecord::Base
belongs_to :subject, polymorphic: true
belongs_to :event, -> { includes(:activities).where(activities: { subject_type: 'Event' }) }, foreign_key: :subject_id
belongs_to :image, -> { includes(:activities).where(activities: { subject_type: 'Image' }) }, foreign_key: :subject_id
end
And now you can do
Activity.includes(:part, event: :guests, image: :tags).order(created_at: :desc).limit(10)
But for eager loading to work, you must use for example
activity.event.guests.first
and not
activity.part.guests.first
So you can probably define a method to use instead of subject
def eager_loaded_subject
public_send(subject.class.to_s.underscore)
end
So now you can have a view with
render partial: :subject, collection: activity
A partial with
# _activity.html.erb
render :partial => 'activities/' + activity.subject_type.underscore, object: activity.eager_loaded_subject
And two (dummy) partials
# _event.html.erb
<p><%= event.guests.map(&:name).join(', ') %></p>
# _image.html.erb
<p><%= image.tags.first.map(&:name).join(', ') %></p>
This will hopefully be fixed in rails 5.0. There is already an issue and a pull request for it.
https://github.com/rails/rails/pull/17479
https://github.com/rails/rails/issues/8005
I have forked rails and applied the patch to 4.2-stable and it works for me. Feel free to use my fork, even though I cannot guarantee to sync with upstream on a regular basis.
https://github.com/ttosch/rails/tree/4-2-stable
You can use ActiveRecord::Associations::Preloader to preload guests and tags linked, respectively, to each of the event and image objects that are associated as a subject with the collection of activities.
class ActivitiesController < ApplicationController
def index
activities = current_user.activities.page(:page)
#activities = Activities::PreloadForIndex.new(activities).run
end
end
class Activities::PreloadForIndex
def initialize(activities)
#activities = activities
end
def run
preload_for event(activities), subject: :guests
preload_for image(activities), subject: :tags
activities
end
private
def preload_for(activities, associations)
ActiveRecord::Associations::Preloader.new.preload(activities, associations)
end
def event(activities)
activities.select &:event?
end
def image(activities)
activities.select &:image?
end
end
image_activities = Activity.where(:subject_type => 'Image').includes(:subject => :tags).order(created_at: :desc).limit(20)
event_activities = Activity.where(:subject_type => 'Event').includes(:subject => :guests).order(created_at: :desc).limit(20)
activities = (image_activities + event_activities).sort_by(&:created_at).reverse.first(20)
I would suggest adding the polymorphic association to your Event and Guest models.
polymorphic doc
class Event < ActiveRecord::Base
has_many :guests
has_many :subjects
after_create :create_activities
end
class Image < ActiveRecord::Base
has_many :tags
has_many :subjects
after_create :create_activities
end
and then try doing
Activity.includes(:subject => [:event, :guest]).order(created_at: :desc).limit(20)
Does this generate a valid SQL query or does it fail because events can't be JOINed with tags and images can't be JOINed with guests?
class Activity < ActiveRecord::Base
self.per_page = 10
def self.feed
includes(subject: [:guests, :tags]).order(created_at: :desc)
end
end
# in the controller
Activity.feed.paginate(page: params[:page])
This would use will_paginate.
I am building a public activity stream which contains a stream of the following:
User posted 3 minutes ago
User starred a post
I am using the public_activity gem for achieving this.
My question is whether there is a way to use the includes for a polymorpic function.
The code that i am running currently is as follows:
#app/models/post.rb
class Post < ActiveRecord::Base
include PublicActivity::Common
attr_accessible :content, :visibility
validates_presence_of :content
belongs_to :postable, :polymorphic => true
has_many :stars
end
#app/models/star.rb
class Star < ActiveRecord::Base
include PublicActivity::Common
validate :duplicate_star
belongs_to :user
belongs_to :post
private
def duplicate_star
errors.add(:base, "Post already starred") if Star.exists?(:user_id => self.user, :post_id => self.post)
end
end
#app/controllers/users_controller.rb
class UsersController < ApplicationController
def index
#post = Post.new
#activities = PublicActivity::Activity.order("id desc").all.includes(:trackable, :owner)
end
end
The trackable can be a post or a star.
I have render functions for displaying both.
The problem is, if I try to output something like {user} starred {postcontent}, it does it this way:
activity.trackable.post.content
So this results in multiple queries, each time it finds a post.
How do I tackle this problem/situation?
Thanks in advance.
Are you not able to use the standard eager loading syntax?
#activities = PublicActivity::Activity.order("id desc").includes(:owner, :trackable => {:post => :content})
I need to link Comments to a Post. However the Comment could be (user generated) a simple text, (system generated) a link or an (system generated) image.
At first they all shared the same attributes. So I just needed to create a category attribute, and do different stuff with the text attribute based on that category.
example:
class Comment < ActiveRecord::Base
belongs_to :post
belongs_to :author, :class_name => "User"
CATEGORY_POST = "post"
CATEGORY_IMAGE = "image"
CATEGORY_LINK = "link"
validates :text, :author, :category, :post, :presence => true
validates_inclusion_of :category, :in => [CATEGORY_POST, CATEGORY_IMAGE, CATEGORY_LINK]
attr_accessible :author, :text, :category, :post
def is_post?
self.category == CATEGORY_POST
end
def is_link?
self.category == CATEGORY_LINK
end
def is_image?
self.category == CATEGORY_IMAGE
end
end
However this wil not suffice now, because I doesn't feel clean to dump every value in a generic "text" property. So I was thinking about create a polymorphic model (and if needed in a factory pattern). But when I googled about polymorphic models, I get examples like a Comment on a Post, but the same Comment on a Page, kind of relations. Is my understanding of polymorphic different (a model that acts different in different situations, compared to a model that acts the same under different scopes)?
So how would I set up this kind of relationship?
I was thinking of (and please correct me)
Post
id
Comment
id
post_id
category (a enum/string or integer)
type_id (references either PostComment, LinkComment or ImageComment based on category)
author_id
PostComment
id
text
LinkComment
id
link
ImageComment
id
path
User (aka Author)
id
name
But I have no clue how to setup the model so that I can call post.comments (or author.comments) to get all comments. A nice to have would be that the creation of a comment would be through comment and not link/image/postcomment (comment acting as the factory)
My main question is, how to setup up the activerecord models, so the relations stay intact (a author has comments and a post has comments. Comments being either a Link, Image or Postcomment)
I'm going to answer only your main question, the model setup. Given the columns and tables you used in your question, with the exception of Comment, you can use the following setup.
# comment.rb
# change category to category_type
# change type_id to category_id
class Comment < ActiveRecord::Base
belongs_to :category, polymorphic: true
belongs_to :post
belongs_to :author, class_name: 'User'
end
class PostComment < ActiveRecord::Base
has_one :comment, as: :category
end
class LinkComment < ActiveRecord::Base
has_one :comment, as: :category
end
class ImageComment < ActiveRecord::Base
has_one :comment, as: :category
end
with that setup, you can do the following.
>> post = Post.first
>> comments = post.comments
>> comments.each do |comment|
case comment.category_type
when 'ImageComment'
puts comment.category.path
when 'LinkComment'
puts comment.category.link
when 'PostComment'
puts comment.category.text
end
end
Let's say you have the following models:
class User < ActiveRecord::Base
has_many :comments, :as => :author
end
class Comment < ActiveRecord::Base
belongs_to :user
end
Let's say User has an attribute name, is there any way in Ruby/Rails to access it using the table name and column, similar to what you enter in a select or where query?
Something like:
Comment.includes(:author).first.send("users.name")
# or
Comment.first.send("comments.id")
Edit: What I'm trying to achieve is accessing a model object's attribute using a string. For simple cases I can just use object.send attribute_name but this does not work when accessing "nested" attributes such as Comment.author.name.
Basically I want to retrieve model attributes using the sql-like syntax used by ActiveRecord in the where() and select() methods, so for example:
c = Comment.first
c.select("users.name") # should return the same as c.author.name
Edit 2: Even more precisely, I want to solve the following problem:
obj = ANY_MODEL_OBJECT_HERE
# Extract the given columns from the object
columns = ["comments.id", "users.name"]
I don't really understand what you are trying to achieve. I see that you are using polymorphic associations, do you need to access comment.user.name while having has_many :comments, :as => :author in your User model?
For you polymorphic association, you should have
class Comment < ActiveRecord::Base
belongs_to :author, :polymorphic => true
end
And if you want to access comment.user.name, you can also have
class Comment < ActiveRecord::Base
belongs_to :author, :polymorphic => true
belongs_to :user
end
class User < ActiveRecord::Base
has_many :comments, :as => :author
has_many :comments
end
Please be more specific about your goal.
I think you're looking for a way to access the user from a comment.
Let #comment be the first comment:
#comment = Comment.first
To access the author, you just have to type #comment.user and If you need the name of that user you would do #comment.user.name. It's just OOP.
If you need the id of that comment, you would do #comment.id
Because user and id are just methods, you can call them like that:
comments.send('user').send('id')
Or, you can build your query anyway you like:
Comment.includes(:users).where("#{User::columns[1]} = ?", #some_name)
But it seems like you're not doing thinks really Rails Way. I guess you have your reasons.
Here are my models:
class Lesson < ActiveRecord::Base
belongs_to :topic, :polymorphic => true
validates_presence_of :topic_type, :topic_id
end
class Subject < ActiveRecord::Base
has_many :lessons, :as => :topic
end
class Category < ActiveRecord::Base
has_many :lessons, :as => :topic
end
Now, what I need is a form that will allow the user to create or update Lessons. The questions is, how can I provide a select menu that offers a mix of Subjects and Categories? (To the user, on this particular form, Subjects and Categories are interchangeable, but that's not the case elsewhere.)
Ideally, this would look something like this:
views/lessons/_form.html.haml
= simple_form_for(#lesson) do |f|
= f.input :title
= f.association :topic, :collection => (#subjects + #categories)
That won't work because we'd only be specifying the topic_id, and we need the topic_types as well. But how can we specify those values?
I guess the crux of the problem is that I really want a single select menu that specifies two values corresponding to two different attributes (topic_id and topic_type). Is there any elegant railsy way to do this?
A few notes:
a) Single table inheritance would make this issue go away, but I'd like to avoid this, as Categories and Subjects have their own relationship… I'll spare you the details.
b) I might could pull some javascript shenanigans, yes? But that sounds messy, and if there's a cleaner way to do it, some magic form helper or something, then that's certainly preferable.
c) Though I'm using simple_form, I'm not wedded to it, in case that's complicating matters.
Thanks
If you don't wish to use STI, you can do something similar: create a new model Topic(name:string) which will polymorphically reference Subject or Category.
class Lesson < ActiveRecord::Base
belongs_to :topic
validates_presence_of :topic_id
end
class Topic < ActiveRecord::Base
belongs_to :topicable, :polymorphic => true
end
class Subject < ActiveRecord::Base
has_one :topic, :as => :topicable
has_many :lessons, :through => :topic
accepts_nested_attributes_for :topic
end
class Category < ActiveRecord::Base
has_one :topic, :as => :topicable
has_many :lessons, :through => :topic
accepts_nested_attributes_for :topic
end
In the view where you create a new Subject/Category:
<%= form_for #subject do |subject_form| %>
<%= subject_form.fields_for :topic do |topic_fields| %>
<%= topic_fields.text_field :name %>
<% end %>
<% end %>
After thinking this through, the less dirty implementation IMO would be to hire the JS shenanigans (b):
= f.input_field :topic_type, as: :hidden, class: 'topic_type'
- (#subjects + #categories).each do |topic|
= f.radio_button :topic_id, topic.id, {:'data-type' => topic.class.name, class: 'topic_id'}
With a sprinkle of JS (your needs may vary):
$('input:radio.topic_id').change(function() {
$('input:hidden.topic_type').val($(this).attr('data-type'));
});
Notes:
I use a radio button to select a topic (category or subject) in a list
The class name of each of possible topic is stored in an attribute 'data-type'
When a radio button is selected, the class name is copied to the hidden input via JS
Using: HTML5, jQuery, haml, simple_form