Undefined error when using self.calculate

Question

I want to execute the mysql GROUP_CONCAT function in rails. I use the method of calculating the active record. like this self.calculate (: group_concat ,: id)

I do not know if this is correct.

any idea on how to execute group_concat in rails? along with searching for the activerecord method.

Answer 1

As @Camway notes above, this is easy to do using the correct Rails methodology for JOINING, SELECT, and GROUPing. For example, suppose I have users and regions, a user can have areas from 0 to many, and a region can have users from 0 to many.

Here is my Model Region :

class Region < ActiveRecord::Base # attributes: id (integer), code (string) has_and_belongs_to_many :users end 

Here is my user model:

 class User < ActiveRecord::Base # attributes: id (integer), email (string) has_and_belongs_to_many :regions end 

There is, of course, a table regions_users , with integer fields region_id and user_id.

To get a general-purpose GROUP_CONCAT job that pulls the codes of all the regions to which each user is attached, I just need to add a class method like this to the User model:

 class User < ActiveRecord::Base # attributes: id (integer), email (string) has_and_belongs_to_many :regions class << self def regions_listing joins(:regions) .select("DISTINCT users.email, GROUP_CONCAT(DISTINCT regions.region_code ORDER BY regions.region_code) AS regions_list") .group("users.email") .order("users.email") end end end 

So, with just this code code, everything pulls all users sorted by email.

 ruby > User.regions_listing => [#<User email: " [e-mail] ">,#<User email: " [e-mail] ">,#<User email: " [e-mail] ">,#<User email: " [e-mail] ">,#<User email: " [e-mail] ">] 

Each of these returned objects has an attribute of the #regions_list attribute, which will provide you with a group list of codes for the regions attached to this user through the regions_users table.

This can be seen with a simple #map call:

 ruby > User.regions_listing.map { |u| [u.email, u.regions_list] } => [[" [e-mail] ", "0,1,2,3,4,5"], [" [e-mail] ", "1,2,5"], [" [e-mail] ", "0,4"], [" [e-mail] ", "3"], [" [e-mail] ", "2,3,4,5"]] 

Note that since this uses the appropriate AR support AR methods, this is a chain . That is, you can add “. Region_listing” at the end of the entire AR request to the user model, and it will provide you with a group method / data for any user objects selected by your request up.

In this way:

 ruby > User.where("users.email like 'b%'").regions_listing.map { |u| [u.email, u.regions_list] } => [[" [e-mail] ", "0,4"], [" [e-mail] ", "2,3,4,5"]] 

And you can also get data in your manufactured #regions_list field using HAVING, for example, to find all users connected to both region 0 and region 4:

 ruby > User.regions_listing.having("regions_list LIKE '%0%4%'").map { |u| [u.email, u.regions_list] } => [[" [e-mail] ", "0,1,2,3,4,5"], [" [e-mail] ", "0,4"]]