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)
Related
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 have two models with the appropriate foreign key created in the people table:
class Person < ActiveRecord::Base
belongs_to :family
class Family < ActiveRecord::Base
has_many :people
If I do the following I get an object - #family_members - as an instance variable and I have no problems:
#family_members = Family.find(1)
I can access the 'child' people table fields easily in my view:
#family_members.people.first_name
However, if I use the arel way with "where" etc. I get an "ActiveRecord::Relation", not a normal object, which leaves me stumped as to how to access the same "first_name" field form the people table like I accessed above:
#family_members = Family.where(:id => 1)
or even
#family_members = Family.joins(:people).where(:id => 1)
(is the "joins" even required??)
I understand that using ".first" will cause the query to run:
#family_members = Family.where(:id => 1).first
But it returns an array, not an object, so if I use in my view:
#family_members.people.first_name
I get a "method 'people' unknown" error.
How can I access the 'first_name' field of the people table like I did with the object created by "find" but using an ActiveRecord relation?
* added information 7/15 ********
To clarify what I am looking for -- here is what I would have written if I were writing SQL instead of Arel:
SELECT f.home_phone, f.address, p.first_name, p.last_name, p.birthday
FROM families f INNER JOIN people p ON p.family.id = f.id WHERE family_id = 1
With that query's results loaded into a result set I could access:
myResultSet("home_phone") -- the home_phone from the families table
myResultSet("address") -- the address from the families table
myResultSet("first_name") -- the first_name from the people table
myResultSet("birthdate") -- the birthdate from the people table
If the two tables in the query have a same-named field I would just use "AS" to request one of the fields by another name.
I have used this kind of query/result set for many years in web apps and I am trying to deduce how to do the same in Rails and ActiveRecord.
#family_members.people.first_name shouldn't ever work so I'm surprised you find it working ... #family_members contains a Family object, #family_members.people is an array of Person objects.
The fact that you're calling it #family_members seems to make me think you're expecting it to be an array of Persons... in which case the correct code would be...
#family_members = Family.find(1).people # finds people in first Family object
If you expect #family_members to contain just the first family member, then...
#family_members = Family.find(1).people.first
If you want an array of first names of all family members, then...
#family_members = Family.find(1).people # finds people in 1st Family object
#family_members.map {|member| member.first_name} # array of first_name
#family_members = Family.find(1) and #family_members = Family.where(:id => 1) are functionally identical.. both retrieve the first Family object in the database in each case may contain zero, one, or multiple people.
Just to be clear, the "1" in all examples above refer to which Family object is retrieved, not which Person in the Family.
Here is the situation.
user embed_one profile
profile belongs_to city
I have populated a city table with
id as Integer
name as String
Now I am doing
user.update_attributes(:profile_attributes{:city_id=>"5"})simulating a browser form submission. Then I check user.profile I see that city_id is stored as string. This renders my user.profile.city to give nil.
I am wondering what is the right thing to do here. Should I let my city id be string or BSON object? Or should I try to intercept update_attributes to make mongoid store city_id as integer? The reason I am using Integer as id for city is because I thought searching through Integer is faster than searching through string. And also I have state and city tables and I want to match the ids in a predictable manner so I don't want to use BSON randome key.
Surely, if you use Mongoid, the right way is to use BSON objects as ids. But if you absolutely need to use integers as city ids, you can simulate belongs_to with code like this
class Profile
def city
City.where(:id => self.city_id).last
end
def city=(new_city)
self.city_id = new_city.id
end
end
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!
How can I interact with objects I've created based on their given attributes in Ruby?
To give some context, I'm parsing a text file that might have several hundred entries like the following:
ASIN: B00137RNIQ
-------------------------Status Info-------------------------
Upload created: 2010-04-09 09:33:45
Upload state: Imported
Upload state id: 3
I can parse the above with regular expressions and use the data to create new objects in a "Product" class:
class Product
attr_reader :asin, :creation_date, :upload_state, :upload_state_id
def initialize(asin, creation_date, upload_state, upload_state_id)
#asin = asin
#creation_date = creation_date
#upload_state = upload_state
#upload_state_id = upload_state_id
end
end
After parsing, the raw text from above will be stored in an object that look like this:
[#<Product:0x00000101006ef8 #asin="B00137RNIQ", #creation_date="2010-04-09 09:33:45 ", #upload_state="Imported ", #upload_state_id="3">]
How can I then interact with the newly created class objects? For example, how might I pull all the creation dates for objects with an upload_state_id of 3? I get the feeling I'm going to have to write class methods, but I'm a bit stuck on where to start.
You would need to store the Product objects in a collection. I'll use an array
product_collection = []
# keep adding parse products into the collection as many as they are
product_collection << parsed_product_obj
#next select the subset where upload_state_ud = 3
state_3_products = product_collection.select{|product| product.upload_state_id == 3}
attr reader is a declarative way of defining properties/attributes on your product class. So you can access each value as obj.attribute like I have done for upload_state_id above.
select selects the elements in the target collection, which meet a specific criteria. Each element is assigned to product, and if the criteria evaluates to true is placed in the output collection.