I am trying to get a list, and I will use books as an example.
class Book < ActiveRecord::Base
belongs_to :type
has_and_belongs_to_many :genres
end
class Genre < ActiveRecord::Base
has_and_belongs_to_many :books
end
So in this example I want to show a list of all Genres, but it the first column should be the type. So, if say a genre is "Space", the types could be "Non-fiction" and "Fiction", and it would show:
Type Genre
Fiction Space
Non-fiction Space
The Genre table has only "id", "name", and "description", the join table genres_books has "genre_id" and "book_id", and the Book table has "type_id" and "id". I am having trouble getting this to work however.
I know the sql code I would need which would be:
SELECT distinct genres.name, books.type_id FROM `genres` INNER JOIN genres_books ON genres.id = genres_books.genre_id INNER JOIN books ON genres_books.book_id = books.id order by genres.name
and I found I could do
#genre = Genre.all
#genre.each do |genre|
#type = genre.book.find(:all, :select => 'type_id', :group => 'type_id')
#type.each do |type|
and this would let me see the type along with each genre and print them out, but I couldn't really work with them all at once. I think what would be ideal is if at the Genre.all statement I could somehow group them there so I can keep the genre/type combinations together and work with them further down the road. I was trying to do something along the lines of:
#genres = Genre.find(:all, :include => :books, :select => 'DISTINCT genres.name, genres.description, books.product_id', :conditions => [Genre.book_id = :books.id, Book.genres.id = :genres.id] )
But at this point I am running around in circles and not getting anywhere. Do I need to be using has_many :through?
The following examples use your models, defined above. You should use scopes to push associations back into the model (alternately you can just define class methods on the model). This helps keep your record-fetching calls in check and helps you stick within the Law of Demeter.
Get a list of Books, eagerly loading each book's Type and Genres, without conditions:
def Book < ActiveRecord::Base
scope :with_types_and_genres, include(:type, :genres)
end
#books = Book.with_types_and_genres #=> [ * a bunch of book objects * ]
Once you have that, if I understand your goal, you can just do some in-Ruby grouping to corral your Books into the structure that you need to pass to your view.
#books_by_type = #books.group_by { |book| book.type }
# or the same line, more concisely
#books_by_type = #books.group_by &:type
#books_by_type.each_pair do |type, book|
puts "#{book.genre.name} by #{book.author} (#{type.name})"
end
Related
Using Rails 3.2. My models are nested in such a way:
Review => Reviewable (Country or Shop)
Country => CountryDay => Shop => Photos
Shop => Photos
I have the following:
#reviews = #user.reviews.includes(:user, :reviewable)
Usually we can include nested polymorphic in such a way:
# this will return errors because :shop is not found in the model Shop (:reviewable is actually :shop)
#reviews = #user.reviews.includes(:user, :reviewable => [:shop])
# this will return errors because :photos is not directly associated to Country
#reviews = #user.reviews.includes(:user, :reviewable => :photos)
There are many other variants. How do I work around it to have the ActiveRecord includes the correct model based on its association?
I think I had the same problem just now. I got an ActiveRecord::EagerLoadPolymorphicError when trying to load associations. My solution was to put together a custom join statement which I've put into a scope for easier usage.
Untested, but this may get you going:
class Review < ActiveRecord::Base
scope :eagerly_loaded, -> {
joins('
INNER JOIN shops
ON (reviews.reviewable_type = "Shop" AND reviews.reviewable_id = shops.id)
INNER JOIN countries
ON (reviews.reviewable_type = "Country" reviews.reviewable_id = countries.id)
')
}
end
You then use #user.reviews.eagerly_loaded. Make sure to use a matching .group()-statement to only get the ones you need.
I'm trying to use active-record query possible connections between airports.
I described the models I created already in another question here:
n:m self-join with ruby on rails active record
Basically, what I can do now is that:
ny = Airport.create({"city" => "New York"})
la = Airport.create({"city" => "Los Angeles"})
ny.destinations << la
la.destinations << ny
I ran into an issue querying the data I'm looking for, which is quite simple in SQL but I had no luck with active record yet.
ny = Airport.where('city = ?', 'New York')
ny.destinations
returns the correct objects, but all of them.
The SQL query looks like that:
SELECT "airports".* FROM "airports" INNER JOIN "connections" ON "airports"."id" = "connections"."destination_id" WHERE "connections"."airport_id" = 3
I'd like to filter those results by cities starting with "s" for example, so an SQL query could look like that:
SELECT "airports".* FROM "airports" INNER JOIN "connections" ON "airports"."id" = "connections"."destination_id" WHERE "connections"."airport_id" = 3 AND airports"."city" LIKE "s%"
I tried it this way:
ny.destinations.where('city LIKE ?', '#{params[:query]}%')
But I always get an empty result.
How could I use active record to filter my resulting objetcs?
edit: Thats the best solution I found so far:
I added the cityLike() method to the Airport model:
app/models/airport.rb:
class Airport < ActiveRecord::Base
attr_accessible :city, :name
has_many :connections
has_many :destinations, :through => :connections
has_many :inverse_connections, :class_name => "Connection", :foreign_key => "destination_id"
has_many :inverse_destinations, :through => :inverse_connections, :source => :airport
def self.cityLike(query)
where("city LIKE ?", "%#{query}%")
end
end
app/model/connection.rb:
class Connection < ActiveRecord::Base
attr_accessible :destination_id, :airport_id
belongs_to :airport
belongs_to :destination, :class_name => "Airport"
end
Now I can query the objects with the following statement:
Airport.find(1).destinations.cityLike("a")
Not sure if it's the best solution, but it produces the query I was looking for.
Thanks a lot to all af you!
ActiveRecord::Base.execute(sql) lets you use pure SQL to do your query and returns the relevant model.
What about this?
Airport.find(:all, joins: "INNER JOIN `connections` ON airports.id = connections.destination_id").where("connections.airport_id = ? AND airports.city LIKE ?", ny_id, "s%")
This code:
ny.destinations.where('city LIKE ?', '#{params[:query]}%')
works like this--first you have an object ny repesenting the city of New York. When you say ".destinations" you have now followed a relation you defined in your model to retrieve all the destinations that you can get to from New York. However, if I'm imagining your database schema correctly, these destinations don't actually have a field called "city"; instead, they have a destination_id, which ties the destination to a particular airport, and it's the airport that has a city associated with it.
So when you query the destination table for 'city LIKE ?', it doesn't find any matching records.
Instead, try
ny.destinations.joins(:airports).where('city LIKE ?', '#{params[:query]}%')
Really been struggling trying to get a group by to work when I have to join to another table. I can get the group by to work when I don't join, but when I want to group by a column on the other table I start having problems.
Tables:
Book
id, category_id
Category
id, name
ActiveRecord schema:
class Category < ActiveRecord::Base
has_many :books
end
class Book < ActiveRecord::Base
belongs_to :category
end
I am trying to get a group by on a count of categories. I.E. I want to know how many books are in each category.
I have tried numerous things, here is the latest,
books = Book.joins(:category).where(:select => 'count(books.id), Category.name', :group => 'Category.name')
I am looking to get something back like
[{:name => fiction, :count => 12}, {:name => non-fiction, :count => 4}]
Any ideas?
Thanks in advance!
How about this:
Category.joins(:books).group("categories.id").count
It should return an array of key/value pairs, where the key represents the category id, and the value represents the count of books associated with that category.
If you're just after the count of books in each category, the association methods you get from the has_many association may be enough (check out the Association Basics guide). You can get the number of books that belong to a particular category using
#category.books.size
If you wanted to build the array you described, you could build it yourself with something like:
array = Categories.all.map { |cat| { name: cat.name, count: cat.books.size } }
As an extra point, if you're likely to be looking up the number of books in a category frequently, you may also want to consider using a counter cache so getting the count of books in a category doesn't require an additional trip to the database. To do that, you'd need to make the following change in your books model:
# books.rb
belongs_to :category, counter_cache: true
And create a migration to add and initialize the column to be used by the counter cache:
class AddBooksCountToCategories < ActiveRecord::Migration
def change
add_column :categories, :books_count, :integer, default: 0, null: false
Category.all.each do |cat|
Category.reset_counters(cat.id, :books)
end
end
end
EDIT: After some experimentation, the following should give you close to what you want:
counts = Category.joins(:books).count(group: 'categories.name')
That will return a hash with the category name as keys and the counts as values. You could use .map { |k, v| { name: k, count: v } } to then get it to exactly the format you specified in your question.
I would keep an eye on something like that though -- once you have a large enough number of books, the join could slow things down somewhat. Using counter_cache will always be the most performant, and for a large enough number of books eager loading with two separate queries may also give you better performance (which was the reason eager loading using includes changed from using a joins to multiple queries in Rails 2.1).
Example
class User
has_many :tickets
end
I want to create association which contains logic of count tickets of user and use it in includes (user has_one ticket_count)
Users.includes(:tickets_count)
I tried
has_one :tickets_count, :select => "COUNT(*) as tickets_count,tickets.user_id " ,:class_name => 'Ticket', :group => "tickets.user_id", :readonly => true
User.includes(:tickets_count)
ArgumentError: Unknown key: group
In this case association query in include should use count with group by ...
How can I implement this using rails?
Update
I can't change table structure
I want AR generate 1 query for collection of users with includes
Update2
I know SQL an I know how to select this with joins, but my question is now like "How to get data" . My question is about building association which I can use in includes. Thanks
Update3
I tried create association created like user has_one ticket_count , but
looks like has_one doesn't support association extensions
has_one doesn't support :group option
has_one doesn't support finder_sql
Try this:
class User
has_one :tickets_count, :class_name => 'Ticket',
:select => "user_id, tickets_count",
:finder_sql => '
SELECT b.user_id, COUNT(*) tickets_count
FROM tickets b
WHERE b.user_id = #{id}
GROUP BY b.user_id
'
end
Edit:
It looks like the has_one association does not support the finder_sql option.
You can easily achieve what you want by using a combination of scope/class methods
class User < ActiveRecord::Base
def self.include_ticket_counts
joins(
%{
LEFT OUTER JOIN (
SELECT b.user_id, COUNT(*) tickets_count
FROM tickets b
GROUP BY b.user_id
) a ON a.user_id = users.id
}
).select("users.*, COALESCE(a.tickets_count, 0) AS tickets_count")
end
end
Now
User.include_ticket_counts.where(:id => [1,2,3]).each do |user|
p user.tickets_count
end
This solution has performance implications if you have millions of rows in the tickets table. You should consider filtering the JOIN result set by providing WHERE to the inner query.
You can simply use for a particular user:
user.tickets.count
Or if you want this value automatically cached by Rails.
Declare a counter_cache => true option in the other side of the association
class ticket
belongs_to :user, :counter_cache => true
end
You also need a column in you user table named tickets_count.
With this each time you add a new tickets to a user rails will update this column so when you ftech your user record you can simply accs this column to get the ticket count without additional query.
Not pretty, but it works:
users = User.joins("LEFT JOIN tickets ON users.id = tickets.user_id").select("users.*, count(tickets.id) as ticket_count").group("users.id")
users.first.ticket_count
What about adding a method in the User model that does the query?
You wouldn't be modifying the table structure, or you can't modify that either?
How about adding a subselect scope to ApplicationRecord:
scope :subselect,
lambda { |aggregate_fn, as:, from:|
query = self.klass
.select(aggregate_fn)
.from("#{self.table_name} _#{self.table_name}")
.where("_#{self.table_name}.id = #{self.table_name}.id")
.joins(from)
select("(#{query.to_sql}) AS #{as}")
}
Then, one might use the following query:
users = User.select('users.*').subselect('COUNT(*)', as: :tickets_count, from: :tickets)
users.first.ticket_count
# => 5
I have three models:
class Book < ActiveRecord::Base
has_many :collections
has_many :users, :through => :collections
end
class User < ActiveRecord::Base
has_many :collections
has_many :books, :through => :collections
end
class Collection < ActiveRecord::Base
belongs_to :book
belongs_to :user
end
I'm trying to display a list of the books and have a link to either add or remove from the user's collection. I can't quite figure out the best syntax to do this.
For example, if I do the following:
Controller
class BooksController < ApplicationController
def index
#books = Book.all
end
end
View
...
<% if book.users.include?(current_user) %>
...
or obviously the inverse...
...
<% if current_user.books.include?(book) %>
...
Then queries are sent for each book to check on that include? which is wasteful. I was thinking of adding the users or collections to the :include on the Book.all, but I'm not sure this is the best way. Effectively all I need is the book object and just a boolean column of whether or not the current user has the book in their collection, but I'm not sure how to forumlate the query in order to do that.
Thanks in advance for your help.
-Damien
I have created a gem(select_extra_columns) for returning join/calculated/aggregate columns in a ActiveRecord finders. Using this gem, you will be able to get the book details and the flag indicating if the current user has the book in one query.
In your User model register the select_extra_columns feature.
class Book < ActiveRecord::Base
select_extra_columns
has_many :collections
has_many :users, :through => :collections
end
Now in your controller add this line:
#books = Book.all(
:select => "books.*, IF(collections.id, 1, 0) AS belongs_to_user",
:extra_columns => {:belongs_to_user => :boolean},
:joins => "LEFT OUTER JOIN collections
ON book.id = collections.book_id AND
collections.user_id = #{current_user.id}"
)
Now in your view you can do the following.
book.belongs_to_user?
You're going to to want 2 SQL queries, and O(1) based lookups (probably irrelevant, but it's the principle) to check if they have the book.
The initial calls.
#books = Book.all
#user = User.find(params[:id], :include => :collections)
Next, you're going to want to write the books the user has into a hash for constant time lookup (if people won't ever have many books, just doing an array.include? is fine).
#user_has_books = Hash.new
#user.collections.each{|c|#user_has_books[c.book_id] = true}
And on the display end:
#books.each do |book|
has_book = #user_has_books.has_key?(book.id)
end
I'd err away from caching the book_ids on the user object, simply because going this route can have some funny and unexpected consequences if you ever start serializing your user objects for whatever reason (i.e. memcached or a queue).
Edit: Loading intermediary collection instead of double loading books.
Essentially you need to make one call to get the book information and the Boolean flag indicating if the current user has the book. ActiveRecord finders doesn't allow you to return the join results from another table. We work around this problem by doing a trick.
In your Book model add this method.
def self.extended_book
self.columns # load the column definition
#extended_user ||= self.clone.tap do |klass|
klass.columns << (klass.columns_hash["belongs_to_user"] =
ActiveRecord::ConnectionAdapters::Column.new(
"belongs_to_user", false, "boolean"))
end # add a dummy column to the cloned class
end
In your controller use the following code:
#books = Book.extended_book.all(
:select => "books.*, IF(collections.id, 1, 0) AS belongs_to_user",
:joins => "LEFT OUTER JOIN collections
ON book.id = collections.book_id AND
collections.user_id = #{current_user.id}"
)
Now in your view you can do the following.
book.belongs_to_user?
Explanation:
In the extended_book method you are creating a copy of Book class and adding a dummy column belongs_to_user to the hash. During the query extra join column is not rejected as it exists in the columns_hash. You should use the extended_book only for querying.
If you use it for CRUD operations DB will throw error.
I would first create an instance method in the User model that 'caches' the all the Book ID's in his collection:
def book_ids
#book_ids ||= self.books.all(:select => "id").map(&:id)
end
This will only execute the SQL query once per controller request. Then create another instance method on the User model that takes a book_id as a parameter and checks to see if its included in his book collection.
def has_book?(book_id)
book_ids.include?(book_id)
end
Then while you iterate through the books:
<% if current_user.has_book?(book.id) %>
Only 2 SQL queries for that controller request :)
Use exists? on the association as it is direct SQL call. The association array is NOT loaded to perform these checks.
books.users.exists?(current_user)
This is the SQL executed by Rails.
SELECT `users`.id FROM `users`
INNER JOIN `collections` ON `users`.id = `collections`.user_id
WHERE (`users`.`id` = 2) AND ((`collections`.book_id = 1)) LIMIT 1
In the above SQL current_user id = 2 and book id is 1
current_user.books.exists?(book)
This is the SQL executed by Rails.
SELECT `books`.id FROM `books`
INNER JOIN `collections` ON `books`.id = `collections`.book_id
WHERE (`books`.`id` = 3) AND ((`collections`.user_id = 4)) LIMIT 1
In the above SQL current_user id = 4 and book id is 3
For more details, refer to the documentation of the exists? method in a :has_many association.
Edit: I have included additional information to validate my answer.