activerecord - Rails Eager Load Identifiers in has_many Association -

say have following models:

class category < activerecord::base     has_and_belongs_to_many :items end  class item < activerecord::base     has_and_belongs_to_many :categories end 

i'm building endpoint retrieves items, each item should coupled array of category ids belongs to. example result be:

[     {         name: 'my item',         category_ids: [1, 2, 3]     },     // more items... ] 

with configuration, i'm able call item.category_ids on each record results in sql want, looks select categories.id categories inner join categories_items ... categories_items.item_id = $1.

however, results in n+1 - , can't seem find way in eager load. i'm looking list of items, , just id of categories they're in, eager loading category_ids field of each item (although, if there's way accomplish same using eager loading i'm okay well)

edit explain difference duplicate:

i'm looking see if it's possible 2 in 2 separate queries. 1 fetches items, , 1 fetches category ids. items table relatively wide, i'd prefer not multiply number of rows returned number of categories each item has - if possible.

using join model instead of has_and_belongs_to_many allow query join table directly instead (without hacking out query yourself):

class category < activerecord::base   has_many :item_categories   has_many :items, through: :item_categories end  class item < activerecord::base   has_many :item_categories   has_many :categories, through: :item_categories end  class itemcategory   belongs_to :item   belongs_to :category end 

note table naming scheme has_many through: different - instead of items_categories item_categories. constant lookup work correctly (otherwise rails looks items::category join model!) - need migration rename table.

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items = item.all category_ids = itemcategory.where(item_id: items.ids).pluck(:category_id)