ActiveRecord has_many through chained filter

Question

I have projects with many materials and I am trying to make a chain of scope to filter them:

project1 associated with paper . project2 associated with plastic

if the user click on paper I want that project1 and project2 are displayed. If after this first query user click on plastic I want to filter the first query result with the new constraint.

In other word if I select paper and plastic I only want to have project2 displayed.

My models:

Project model

class Project < ApplicationRecord
    has_many :project_materials
    has_many :materials, through: :project_materials

    scope :with_materials, -> (materials) { includes(:materials).where(materials: {id: materials} ) }
end

Material Model:

class Material < ApplicationRecord
  has_many :project_materials
  has_many :projects, through: :project_materials
end

project_material Model:

class ProjectMaterial < ApplicationRecord
  belongs_to :project
  belongs_to :material
end

I tried the following in the rails console:

p1 = Project.first
p2 = Project.last
m1 = Material.first
m2 = Material.last

p1.materials << m1

p2.materials << m1
p2.materials << m2

f1 = Project.with_materials(m1)
# which return p1 and p2

f2 = f1.with_materials(m2)
# which return nothings because of the following query

'SQL (1.0ms)  SELECT "projects"."id" AS t0_r0, "projects"."name" AS t0_r1, "projects"."difficulty" AS t0_r2, "projects"."status" AS t0_r3, "projects"."duration" AS t0_r4, "projects"."uuid" AS t0_r5, "projects"."slug" AS t0_r6, "projects"."created_at" AS t0_r7, "projects"."updated_at" AS t0_r8, "projects"."project_type_id" AS t0_r9, "materials"."id" AS t1_r0, "materials"."name" AS t1_r1, "materials"."created_at" AS t1_r2, "materials"."updated_at" AS t1_r3 FROM "projects" LEFT OUTER JOIN "project_materials" ON "project_materials"."project_id" = "projects"."id" LEFT OUTER JOIN "materials" ON "materials"."id" = "project_materials"."material_id" WHERE "materials"."id" = 5'

thank you in advance.

Answer 1

I have a solution maybe not the most elegant but at least it's working.

def self.with_materials materials
  materials = [materials].flatten.compact

  sql = "SELECT project_id, count(material_id) FROM project_materials WHERE material_id IN (?) GROUP BY project_id HAVING count(material_id) > ?"
  ids = ProjectMaterial.find_by_sql [sql, (materials), (materials.count - 1)]
  ids.collect!{ |i| i.project_id }

  where( id: ids )
end