I always wondered how to query and get results that doesn't fit in a model. Similar how it's done using LINQ and projecting into anonymous objects.
So here's the simple schema:
# Product.rb
class Product < ActiveRecord::Base
has_many :product_views
# attributes: id, name, description, created_at, updated_at
end
# ProductView.rb
class ProductView < ActiveRecord::Base
belongs_to :product
# attributes: id, product_id, request_ip, created_at, updated_at
end
Basically I need to get a list of Products (preferably just id and name) along with the count of views it had. Obviously ordered by view count desc.
This is the SQL I want to get:
select
p.id,
p.name,
count(pv.product_id) as views
from
product_views pv
inner join
products p on pv.product_id = p.id
group by
pv.product_id
order by
count(product_id) desc
I tried the following and similar, but I'm getting ProductView objects, and I would like to get just an array or whatever.
ProductView.includes(:product)
.group('product_id')
.select("products.id, products.name, count(product_id)")
This kind of thing are trivial using plain SQL or LINQ, but I find myself stucked with this kind of queries in Rails. Maybe I'm not thinking in the famous 'rails way', maybe I'm missing something obvious.
So how do you do this kind of queries in Rails 3, and specifically this one? Any suggestions to improve the way I'm doing this are welcome.
Thank you
You can use Arel to do what you're looking for:
products = Product.arel_table
product_views = ProductView.arel_table
# expanded for readability:
sql = products.join(product_views)
.on(product_views[:product_id].eq(product[:id]))
.group(product_views[:product_id])
.order('views DESC')
.project(products[:id],
products[:name],
product_views[:id].count.as('views'))
products_with_views = Product.connection.select_all(sql.to_sql) # or select_rows to just get the values
Yes, it is long, but Arel is a very smart way to deal with creating complex queries that can be reused regardless of the database type.
Within a class method in the Product class:
Product.includes(:product_views).all.map { |p| [p.id, p.name, p.product_views.size] }
Then sort it however you want.
I don't know if there's a way to do it using your models. I would probably resort to:
Product.connection.select_rows(sql)
Which will give you an array of arrays. You can use select_all if you'd rather have an array of hashes.
Try this:
#product = Product.find(#product_id)
#product_views = #product.product_views.count
(Source - http://ar.rubyonrails.org/classes/ActiveRecord/Calculations/ClassMethods.html#M000292)
Hope this helps!
Related
I can't figure out the right syntax to use when including several models and using AND or OR clauses.
For example, there Shop model that has_one relation with Address model and belongs_to with Country.
How for example add OR to the below query:
Shop.includes(:address, :country)
Trying like this:
Shop.includes(:address, :country).where('countries.code'=> 'FR').and('counties.updated_at > ?', Date.today.days_ago(7))
raises the error:
NoMethodError: undefined method `and' for #<Shop::ActiveRecord_Relation:0x00007fb90d0ea3f8>
I found this thread at SO, but in this case, I have to repeat the same where clause before each OR statement? - looks not so DRY :(
What am I missing ?
Don't kick yourself... you don't need to use and at all, just string another where in:
Shop.includes(:address, :country).where('countries.code'=> 'FR').where('counties.updated_at > ?', Date.today.days_ago(7))
There is a better solution if you need to add multiple OR clause to AND clause. To get around it, there is arel_table method that can be used as follows.
Let's say we have the following models
Shop -> has_one :address
Shop -> belongs_to :country
and we would like to find all the shops by country code and address updated_at or country updated_at should be greater then a date you pass in:
some_date = Date.today
countries = Country.arel_table
addresses = Address.arel_table
# creating a predicate using arel tables
multi_table_predicate = countries[:updated_at].gt(some_date).or(addresses[:updated_at].gt(some_date))
# building the query
shops = Shop.includes(:address, :country).where(countries: {code: 'FR'}).where(multi_table_predicate)
This will execute a LEFT OUTER JOIN and here is where clause:
WHERE "countries"."code" = $1 AND ("countries"."updated_at" > '2019-03-12' OR "addresses"."updated_at" > '2019-03-12')
Sure, you can chain more tables and multiply OR conditions if you want.
Hope this helps.
Suppose I've got ActiveRecord models such that User has_one :photo. In the database, photos has a t.binary column which may hold a lot of data, so I don't want to SELECT that column unless I need to.
I want to do something like:
users.each do |user|
image_tag(user_photo_path) if user.photo.present?
end
However, I don't want to call user.photo.present? because:
Doing so loads the photo association, including SELECT * from photos
Even if it could be made to only SELECT id FROM photos to check existence, it's still an N + 1 query.
What I really want is to load users with a single query which gives each one a property telling me whether it has an associated photo or not.
With ActiveRecord 5, this works:
class User < ActiveRecord::Base
scope :with_photo_id, -> {
left_outer_joins(:photo).select(
"users.*, photos.id AS photo_id"
)
}
end
Then I can call User.with_photo_id and check user.photo_id.present?.
Prior to AR 5, the join would be uglier:
joins(
"LEFT OUTER JOIN photos ON photos.user_id = users.id"
)
I am trying to retrieve a child object based on the key in its parent's table. For instance, I have the Customer class which contains a "store_id" key to the Stores tables. If a customer has a "store_id" key, I would like to bring back that Store object and not the parent Customer object.
EDIT: Here is a sql statement showing what I am trying to do.
So the SQL statement would look something like this.
"SELECT storeS.* FROM customers INNER JOIN stores ON customers.store_id = storeS.id WHERE customers.id = '9'"
I know the sql is probably wrong, but thats a very concise way to show it.
I am assuming you are using rails with the out-of-the-box configuration (using ActiveRecord).
By convention, the "store_id" key in the "customers" table should match an "id" field in the "stores" table. You should also have the following class models setup:
class Store < ActiveRecord::Base
has_many :customers # this is not required for what you want to do here, but recommended
end
class Customer < ActiveRecord::Base
belongs_to :store
end
Assuming this is true, you can either do this if you have the store key:
# assuming we have store key == 9
Store.find(key)
Or you could do this if you already have the customer:
# assuming we have customer.store_id == 9
customer.store
Or if you only have the customer key:
# assuming we have a customer key == 9
customer = Customer.find(9)
store = customer.store
I don't use ActiveRecord a lot, but I think it's this:
Store.find(customer.store_id)
Say I have the following in my controller:
#category1
#category2
and I want to find all stores associated with those two categories...
#stores = #category1.stores + #category2.stores
this does work, but unfortunately returns an unaltered Array, rather than a AR::Base Array, and as such, I can't do things like pagination, scope, etc...
It seems to me like there's a built-in way of finding through multiple instance association... isn't there?
##stores = #category1.stores + #category2.stores
#if you want to call API methods you can just add conditions with the category id
#stores = Store.find(:all, :conditions => ['category_id=?', a || b])
With ActiveRecord, whenever you're finding a set of unique model objects, calling find on that model is usually your best bet.
Then all you need to do is constrain the join table with the categories you care about.
#stores = Store.all(:joins => :categories,
:conditions => ['category_stores.category_id in (?)', [#category1.id, #category2.id]])
I have an Author model which habtm :feeds. Using Rails 3 want to setup a scope that finds all authors that have no associated feeds.
class Author < ActiveRecord::Base
has_and_belongs_to_many :feeds
scope :without_feed, joins(:feeds).where("authors_feeds.feed_id is null")
end
...doesn't seem to work. It feels like a simple thing. What am I missing here?
To my knowledge ActiveRecord/Arel do not have a means of defining outer joins. So you'll have to write a bit more SQL than normal. Something like this should do the trick:
scope :without_feed, joins('left outer join authors_feeds on authors.id=authors_feeds.author_id').where('authors_feeds.feed_id is null')
I am of course guessing at your table names and foreign keys. But that should give you the picture.
In Rails >= 5, you can do it like this:
scope :without_feed, -> {
left_outer_joins(:feeds)
.where(authors_feeds: { author_id: nil })
}
scope :with_feed, -> {
left_outer_joins(:feeds)
.where.not(authors_feeds: { author_id: nil })
}