In rails6 API modeFrom the users (free_users) registered in the same station, narrow down the stations (active_stations) where there are 6 or more unscheduled users, and form a chat room with unplanned users who have the same station.I'm trying.

I will put the error statement first.
Error ①

NoMethodError (undefined method `free'for User (call'User.connection' to establish a connection): Class

Error ②

ActiveRecord :: StatementInvalid (PG :: UndefinedTable: ERROR: missing FROM-clause entry for table "free_users"

user_rooms_controller (intermediate table that links chat rooms and users)

def create
#This is the place where the error occurred -------------------------------------------------------------------------------
In case of error ① @active_stations = Station.joins (: users) .where ("users.where ({free: true} .count>= 6)")
In case of error ② @ active_stations = Station.joins (: users) .where ("users.where ({free: true} .count>= 6)")
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    @ active_stations.each do | station | #Processing for each settable station
      room = Room.create (station_id: station.id) # 1, create a room for a station that can be set
      users = station.users.where (free: true) #Get unplanned users who have registered the same settable station
      selected_users = users.sample (6) # Randomly extract 6 people from unplanned users who have registered the same station
      selected_users.each do | user | # Treat unplanned users to have the same room id
        room.save
        user_room = UserRoom.build! (User_id: user.id, room_id: room.id)
        user.free = false
        user.save
        render json: {status:'SUCCESS', message:'Loaded user_rooms', data: user_room}
      end
    end
  end

station.rb (define free_users here)

def free_users
    free_users = self.users.where (free: true)
    return free_users
end

I tampered with the location where the error occurred as follows. I understand that I can't use the separately defined free_users in terms of how to use the where method.

@active_stations = Station.where (user: User.where (free: true))

Thanks for your professor!