Make array of values from table attribute that matched another tables attribute - ruby-on-rails

Question

In my Ruby on Rails 3 app controller, I am trying to make an instance variable array to use in my edit view.
The User table has a user_id and reseller_id.
The Certificate table has a user_id.
I need to get the reseller_id(s) from the User table that have the user_id(s) in both User table and Certificate table.
Here is my User model:
class User < ActiveRecord::Base
attr_accessible :email, :name, :password, :password_confirmation, :remember_token, :reseller_id, :validate_code, :validate_url, :validated, :admin, :avatar
belongs_to :reseller
has_one :certificate
end
Here is my Certificate model:
class Certificate < ActiveRecord::Base
attr_accessible :attend, :pass, :user_id
validates :user_id, presence: true
end
Here is my controller, this seems to only store the last user_id in the Certificate table.
##train should be reseller.id(s) of all users in Certification table.
#certs = Certificate.all
#certs.each do |user|
#id = []
#id << user.user_id
#id.each do |id|
if User.find(id)
#train = []
#train << User.find(id).reseller_id
end
end
end
Thank you

Answer 1

1) Correct version of your code
#certs = Certificate.all
#reseller_id = [] # 1
#certs.each do |user|
id = user.user_id # 2
if u = User.find(id)
#reseller_id << u.reseller_id # 3
end
end
2) Rails way
Something like this
#reseller_id = User.joins(:certificates).select('users.reseller_id').map { |u| u['reseller_id']}
PS
Don't keep this code in controller please :-)

Answer 2

Well, first of all, you should not nest your ids' each block inside of the each block for certificates. You should build your ids array, then loop over it later. The reason you are only getting the last user_id, is because "#id" will only ever have a single element in it as your code is currently written. You will also run into the same problem with your "#train" array. Because you are declaring the array inside the iterator, it is getting re-created (with nothing in it) on every iteration. Using your existing code, this should work:
#certs = Certificate.all
#ids = []
#certs.each do |user|
#ids << user.user_id
end
#train = []
#ids.each do |id|
if User.find(id)
#train << User.find(id).reseller_id
end
end
A more Rubyish and concise way would be the following:
cert_user_ids = Certificate.all.map(&:user_id)
reseller_ids = cert_user_ids.map { |id| User.find(id).reseller_id rescue nil }.compact
Map is an enumerable method that returns an array of equal size to the first array. On each iteration, whatever the code inside the block returns "replaces" that element in the new array that is returned. In other words, it maps the values of one array to a new array of equal size. The first map function gets the user_ids of all certificates (using &:user_id is a shortcut for Certificate.all.map { |cert| cert.user_id } ) The second map function returns the "reseller_id" of the user, if a user is found. It returns nil if no user is found with that id. Finally, compact removes all nil values from the newly mapped array of reseller_ids, leaving just the reseller ids.
If you want to do this in the most efficient and railsy way possible, minimizing database calls and allowing the database to do most of the heavy lifting, you will likely want to use a join:
reseller_ids = User.joins(:certificates).all.map(&:reseller_id)
This grabs all users for which a certificate with that user's id exists. Then it utilizes map again to map the returned users to a new array that just contains user.reseller_id.
Ruby tends to be slower at this type of filtering than RDBM systems (like mysql), so it is best to delegate as much work as possible to the database.
(Note that this join will compare user.id to certificate.user_id by default. So, if your 'primary key' in your users table is named 'user_id', then this will not work. In order to get it to work, you should either use the standard "id" as the primary key, or you will need to specify that 'user_id' is your primary key in the User model)