json_dealer.rb

  def json_dealer
    render json: Customers.
      select('customers.first_name, customers.last_name,
        customers.street, customers.city, customers.zip,
        customers.home_phone, mobile_phone,
        coordinates.latitude, coordinates.longitude, coordinates.distance,
        vehicles.make, vehicles.model, vehicles.year').
      where(['customers.dealership_id = ?', params[:id]] ).
      joins('join coordinates on customers.id = coordinates.customer_id').
      joins('join vehicles on customers.id = vehicles.customer_id').
      where(['vehicles.make = ?', params[:make]]) if params[:make].presence
  end

customers = Customers.
      select('customers.first_name, customers.last_name,
        customers.street, customers.city, customers.zip,
        customers.home_phone, mobile_phone,
        coordinates.latitude, coordinates.longitude, coordinates.distance,
        vehicles.make, vehicles.model, vehicles.year').
      where(['customers.dealership_id = ?', params[:id]] ).
      joins('join coordinates on customers.id = coordinates.customer_id').
      joins('join vehicles on customers.id = vehicles.customer_id')

customers.where!(['vehicles.make = ?', params[:make]]) if params[:make].presence

render json: customers