module SimilarModels

# File lib/similar_models.rb, line 39
def self.join_table_values(many_to_many_assocation)
  if many_to_many_assocation.macro == :has_and_belongs_to_many
    join_table = many_to_many_assocation.join_table
    foreign_key = many_to_many_assocation.foreign_key
    association_foreign_key = many_to_many_assocation.association_foreign_key
  elsif many_to_many_assocation.macro == :has_many
    join_table = many_to_many_assocation.through_reflection.table_name
    foreign_key = many_to_many_assocation.through_reflection.foreign_key
    association_foreign_key = many_to_many_assocation.foreign_key
  end
  [join_table, foreign_key, association_foreign_key]
end

# File lib/similar_models.rb, line 5
def has_similar_models(*many_to_many_associations, as: nil)
  as = "similar_#{model_name.plural}" unless as

  # defaults to 'def similar_{model name}'
  define_method as do
    table_name = self.class.table_name
    primary_key = self.class.primary_key
    primary_key_ref = "#{table_name}.#{primary_key}"
    association_scopes = []

    many_to_many_associations.each do |many_to_many_association|
      assocation = self.class.reflect_on_association(many_to_many_association)
      join_table, foreign_key, association_foreign_key = self.class.join_table_values(assocation)

      association_scopes << self.class.where(
        "#{join_table}.#{association_foreign_key} IN \
        (select #{join_table}.#{association_foreign_key} from #{join_table} \
        where #{join_table}.#{foreign_key} = :foreign_key)", foreign_key: self.id
      ).joins("INNER JOIN #{join_table} ON #{join_table}.#{foreign_key} = #{primary_key_ref}")
    end

    scope = self.class.select("#{table_name}.*, count(#{primary_key_ref}) AS #{as}_model_count").
      where.not(primary_key => self.id).order("#{as}_model_count DESC")
    group_by_clause = self.class.column_names.map { |column| "#{table_name}.#{column}"}.join(', ')

    # if there is only one many-to-many association no need to use UNION sql syntax
    if association_scopes.one?
      scope.merge(association_scopes.first).group(group_by_clause)
    else
      # see http://blog.ubersense.com/2013/09/27/tech-talk-unioning-scoped-queries-in-rails/
      scope.from("((#{association_scopes.map(&:to_sql).join(') UNION ALL (')})) AS #{table_name}").group(group_by_clause)
    end
  end

  def self.join_table_values(many_to_many_assocation)
    if many_to_many_assocation.macro == :has_and_belongs_to_many
      join_table = many_to_many_assocation.join_table
      foreign_key = many_to_many_assocation.foreign_key
      association_foreign_key = many_to_many_assocation.association_foreign_key
    elsif many_to_many_assocation.macro == :has_many
      join_table = many_to_many_assocation.through_reflection.table_name
      foreign_key = many_to_many_assocation.through_reflection.foreign_key
      association_foreign_key = many_to_many_assocation.foreign_key
    end
    [join_table, foreign_key, association_foreign_key]
  end
end