Ruby on Rails活动记录联接

Question

I need to joins this two tables but I cant seems to get it right.

Model

class Registration < ActiveRecord::Base has_many :region end

class Region < ActiveRecord::Base belongs_to :registration end

what I have tried so far is this

`test = Region.joins(:registration)`

which will give me this result

[#<Region id: 1, name: "region1">, #<Region id: 1, name: "region1">]

It seem that i have certainly did join the two tables but my dilemma is I also need the index that belongs to Registration Model. the about results are only the index available in the Region Model.

I also tried it the other way around

test = Registration.joins(:region)

but it gives me a error

Unknown column 'regions.registration_id' in 'on clause': SELECT registrations .* FROM registrations INNER JOIN regions ON regions . registration_id = registrations . id``

and I do agree with this error cause i do not have a index registration_id

Answer 1

when you define belongs_to relationship in a table that table should have foreign_key of table to which it is belonging. So in your example if you have relation like Region belongs to registrations then regions should have registration_id. It looks like you have defined a wrong relationship.

So if you will define registration_id in Region and execute following query:

Region.joins(:registration) 

You will see id of region table along with registration_id in the result

Hope this helps

Answer 2

Change the Registration model code

class Registration < ActiveRecord::Base
  has_many :regions
end

Then you can use joins

test = Registration.joins(:region)

Hope it helps!

Answer 3

class Registration < ActiveRecord::Base
    has_many :regions, inverse_of: :registration, dependent: :destroy
 end

class Region < ActiveRecord::Base
    belongs_to :registration, inverse_of: :regions
end

Then, make sure your Region table correctly shows its association to Registration with a registration_id column. If it doesn't contain the column, go ahead and create the following migration:

rails g migration add_registration_ref_to_regions registration:references

rake db:migrate

Now, you should be able to successfully query via:

Registration.joins(:regions) or Region.joins(:registration)

Cheers!