ActiveRecord - Retrieve attributes from another model - relation does not exist

[Leandro França]

Hi there,

I'm learning the active model, and I'm trying to retrieve an association.

My models are:

class Composition < ActiveRecord::Base
  has_and_belongs_to_many :materials
  belongs_to :product
end
 
class Material < ActiveRecord::Base
 has_and_belongs_to_many :compositions
 has_many :products, through: :composition
end
   
class Product < ActiveRecord::Base
  has_many :compositions
  has_many :materials, through: :composition
  accepts_nested_attributes_for :materials
end

My schema is:

 

 create_table "compositions", force: :cascade do |t|
    t.integer  "product_id"
    t.integer  "material_id"
    t.integer  "material_quantity"
    t.datetime "created_at",        null: false
    t.datetime "updated_at",        null: false
  end


  create_table "materials", force: :cascade do |t|
    t.string   "name"
    t.decimal  "unit_cost"
    t.string   "unit_measure"
    t.datetime "created_at",   null: false
    t.datetime "updated_at",   null: false
  end


  create_table "products", force: :cascade do |t|
    t.string   "name"
    t.string   "description"
    t.datetime "created_at",  null: false
    t.datetime "updated_at",  null: false
  end

On my CompositionsController index method, I would like to retrieve all raw materials for a product id.

What i have  now is:

  def index
    @product = Product.find(params[:product_id])
    @compositions = @product.compositions
  end

How do I retrieve materials attributes from @compositions?

When I try to use  

@compositions = @product.compositions.includes(:materials)

It gives me the error:

PG::UndefinedTable: ERROR:  relation "compositions_materials" does not exist
LINE 5:                WHERE a.attrelid = '"compositions_materials"'...
                                          ^
:               SELECT a.attname, format_type(a.atttypid, a.atttypmod),
                     pg_get_expr(d.adbin, d.adrelid), a.attnotnull, a.atttypid, a.atttypmod
                FROM pg_attribute a LEFT JOIN pg_attrdef d
                  ON a.attrelid = d.adrelid AND a.attnum = d.adnum
               WHERE a.attrelid = '"compositions_materials"'::regclass
                 AND a.attnum > 0 AND NOT a.attisdropped
               ORDER BY a.attnum

Any hints?

Thanks in advance,

Leandro

[Marco Antonio Almeida]

Hi Leandro,

The way it is set up right now, you would actually have in the Composition model a belongs_to :material. And the Material would have a has_many :compositions.

Verify that you have the foreign key set in your Composition model, so you would never be able to do something like: composition.materials.

Since you have the has_many through association you could simply call. @product.materials and it will return what you want (It will do the INNER JOIN through the compositions table).

The error you're getting is because when you're using a has_and_belongs to many, Rails expect to have a relationship table named after the two models in the plural. But in this case your compositions table is already the relationship table.

Best regards,

Marco Almeida

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[Marco Antonio Almeida]

Now I see that you have the quantity inside your compositions table and you probably need that.

You can use the code you have in your controller normally with the changes mentioned in my last e-mail.

Marco Almeida

about.me/marcoafilho
+45 31 65 28 84

[Leandro França]

Hey Marco,

Thanks a lot for you quick response.

So, I tried to change my models to use only has_many and belongs_to, instead of has_and_belongs_to_many.
I also added the foreign key to material_id:

class Composition < ActiveRecord::Base
  belongs_to :material, :class_name => "Material", :foreign_key => 'material_id'

  belongs_to :product
end


class Material < ActiveRecord::Base

 has_many :compositions
 has_many :products, through: :composition

end
   
class Product < ActiveRecord::Base
  has_many :compositions
  has_many :materials, through: :composition

end

On rails console, I can run fine:

@p = Product.first
@p.compositions
@p.compositions.includes(:material)

But this last one returns me only the items I already have on composition, not the actual material attributes.

@p.compositions.includes(:material)
  Composition Load (0.5ms)  SELECT "compositions".* FROM "compositions" WHERE "compositions"."product_id" = $1  [["product_id", 1]]
  Material Load (0.7ms)  SELECT "materials".* FROM "materials" WHERE "materials"."id" IN (1, 2)
=> #<ActiveRecord::AssociationRelation [#<Composition id: 5, product_id: 1, material_id: 1, material_quantity: 3, created_at: "2015-03-30 05:20:27", updated_at: "2015-03-30 05:20:27">, #<Composition id: 6, product_id: 1, material_id: 2, material_quantity: 2, created_at: "2015-03-30 05:20:27", updated_at: "2015-03-30 05:20:27">]>

I would like to also get the material names from Material model.
How do I get them using this approach?

Thanks again,

Leandro

[Marco Antonio Almeida]

Hi Leandro,

You can reach the materials through the compositions, and you should worry about doing extra queries in the DB since you have used the includes method. You should iterate through them to get the list of material names.

Say we have:

@compositions = @p.compositions.include(:materials)

@material_names = @compositions.map { |c| c.materials.map(&:name) }

@material_names will return you an array of arrays with the material names.

Since you have the has many through setup, you can call @product.materials.map(&:name) directly and you will get a flattened array with the materials' names.

Best regards,

Marco

To view this discussion on the web visit https://groups.google.com/d/msgid/rubyonrails-talk/be51416c-4340-4c12-afff-702f67820f0f%40googlegroups.com.

[Leandro França]

Thanks again Marco,

I ended up adding a references to my previous query, and it seems its working fine now:

@compositions = @product.compositions.includes(:material).references(:materials)

Regards,

Leandro