https://www.reddit.com/r/rails/comments/gd4rap/query_problem_possible_on_activerecord_or_low/

customer.rb

class Customer < ApplicationRecord
  has_many :customer_documents
  has_many :documents, through: :customer_documents
end

customer_document.rb

class CustomerDocument < ApplicationRecord
  belongs_to :customer
  belongs_to :document
end

document.rb

class Document < ApplicationRecord
  belongs_to :document_type
  has_many :customer_documents
  has_many :customers, through: :customer_documents
end

document_type.rb

class DocumentType < ApplicationRecord
  has_many :documents
  has_many :customers, -> { distinct }, through: :documents
end