rails active record to fetch result from multiple tables

Question

I have the following query

SubCategory.joins(dropdown_heads: :dropdown_lists).where(id: params[:sub_cat_id])

The above query generates

SELECT "sub_categories".* FROM "sub_categories" INNER JOIN "dropdown_heads" ON "dropdown_heads"."sub_category_id" = "sub_categories"."id" INNER JOIN "dropdown_lists" ON "dropdown_lists"."dropdown_head_id" = "dropdown_heads"."id" WHERE "sub_categories"."id" = 6

But what I actually need is to fetch records from dropdown_heads and dropdown_lists only.

What modification do I need to achieve it?

Following are the associations

sub_category.rb

has_many: dropdown_heads

dropdown_head.rb

has_many: dropdown_lists
belongs_to: sub_category

dropdown_lists.rb

belongs_to: dropdown_head

rb512 · Accepted Answer · 2013-10-27 07:28:13Z

1

Retrieve the sub_category record from the database

@sub_category = SubCategory.find(params[:sub_cat_id])

Retrieve all dropdown_heads

sub_category.dropdown_heads

Iterate through dropdown_head to retrieve dropdown_lists

@sub_category.dropdown_heads {|dropdown_head| dropdown_head.dropdown_lists} #do whatever you want with dropdown_lists

which is same as:

@sub_category.dropdown_heads.each |dropdown_head|
 dropdown_head.dropdown_lists
end

edited Oct 27, 2013 at 7:28

answered Oct 27, 2013 at 6:59

rb512

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9 Comments

rb512 Over a year ago

you only need to get sub_category from the controller. once you have the correct sub_category object, you can do the rest in your views

Prabhakaran Over a year ago

Had this in my view <%= sub_category.dropdown_heads %> but throwing exception undefined local variable or method sub_category'`

rb512 Over a year ago

make sub_category an instance variable (add "@" prefix, see my updated answer)

Prabhakaran Over a year ago

I cant understand what is happening in your third code. Can you please explain it

kristinalim Over a year ago

To reduce the number of SQL queries, you can use eager-loading: @sub_category.dropdown_heads.includes(:dropdown_lists)

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nickcen · Accepted Answer · 2013-10-27 15:03:40Z

0

You don't have to get the sub_category instance, you can just call

    dropdown_heads = DropdownHead.find_all_by_sub_category_id(params[:sub_cat_id])

and the remaining are just the same as @rb512 said.

answered Oct 27, 2013 at 15:03

nickcen

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Comments

Deepak Kabbur · Accepted Answer · 2015-04-16 07:15:41Z

0

You can use

SubCategory.joins(dropdown_heads: :dropdown_lists).where(id: params[:sub_cat_id].select("dropdown_heads.dropdown_heads, dropdown_lists.dropdown_lists")

Even you can use pluck() method of rails to retrieve specific fields.

answered Apr 16, 2015 at 7:15

Deepak Kabbur

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