你如何为 has_many 关系编写 Rails finder 查询？

Question

I'm using Rails 5. I have the following class

class ParentObject < ApplicationRecord
    has_and_belongs_to_many :child_objects, :optional => false
end

I have a parameter, params[:child_objects] , passed in to my controller that is an array of those objects' IDs. How can I write a finder to return objects tied to those IDs? I tried this

parent_objects = ParentObject.joins(:child_objects).where(
  :child_objects => child_objects
)

but got this error

Unknown primary key for table parent_objects_child_objects in model ParentObject::HABTM_ChildObjects

Answer 1

You can find these registers with this query bellow

ParentObject.joins(:child_objects).where('child_objects.id in (?)', child_objects)

where('child_objects.id in (?)', child_objects) # you are searching ids into child_objects table. Specify the join table child_objects.id

This is the same query, but more RailsWay. Same idea

ParentObject.joins(:child_objects).where(child_objects: { id: child_objects} )

Answer 2

Based on the classes defined, has_and_belongs_to_many means a parent can have many children, as well as a child, can belong to many parents. This qualifies for a many to many relation and thus needs an associative (or junction) table.

For above-defined classes, rails will look for child_objects_parent_objects and raise error if table not present. Add a junction table as defined below

CREATE TABLE `child_objects_parent_objects` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `parent_object_id` int(11) NOT NULL,
  `child_object_id` int(11) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 ROW_FORMAT=DYNAMIC;

And then the below query will return parent objects for the given child objects( params[:child_objects] ).

ParentObject.joins(:child_objects).where(child_objects: { id: child_object_ids})

However, If you want a child to belong to only 1 parent then define a has_many relation & junction table will not be required.

class ParentObject < ApplicationRecord
  has_many :child_objects
end

ParentObject.joins(:child_objects).where(child_objects: { id: child_object_ids})