Why can't ActiveRecord work as an array?

Question

Ruby on Rails

Hello Habr!
There is a code that passes some hash to gon:

gon.select_options = Product.all.to_a.inject({}) do |hash, product|
      hash[product.id] = product.materials.to_a.inject({}) do
            |materials_hash, material| materials_hash[material.id] = get_material_data_hash(material)
      end
end

But in the end, this hash is empty, although there are records in the database. What could be the problem?
UPD. Here is the query log, which seems to say that there are values

ActiveRecord::SchemaMigration Load (0.3ms)  SELECT "schema_migrations".* FROM "schema_migrations"
Processing by AdminPanelController#materials_panel as HTML
  Product Load (0.1ms)  SELECT "products".* FROM "products"
  Material Load (0.6ms)  SELECT "materials".* FROM "materials" INNER JOIN "materials_products" ON "materials"."id" = "materials_products"."material_id" WHERE "materials_products"."product_id" = ?  
  Material Load (0.3ms)  SELECT "materials".* FROM "materials" INNER JOIN "materials_products" ON "materials"."id" = "materials_products"."material_id" WHERE "materials_products"."product_id" = ?  
  Material Load (0.2ms)  SELECT "materials".* FROM "materials" INNER JOIN "materials_products" ON "materials"."id" = "materials_products"."material_id" WHERE "materials_products"."product_id" = ?  

Answer 1

The problem is that the block in the inject method must return a new value for the first parameter, i.e. in your case the same Hash instance into which you write the result
. Here is the inject variant that should work

gon.select_options = Product.all.to_a.inject({}) do |hash, product|
      hash[product.id] = product.materials.to_a.inject({}) do |materials_hash, material|
            materials_hash[material.id] = get_material_data_hash(material)
            materials_hash
      end
      hash
end

Or use each_with_object instead of inject