[Rails] 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

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