dotslash
I’m wondering what the syntax will be if I have to save a many-to-many association to the database, which also involves updating the joining model. Basically, I have a multi-room chat app, where users and rooms are associated through the rooms_users table, and this table also has a ‘status’ attribute (for online, offline, etc.).
Supposing the schema are:
schema "users" do
# The rooms the user is currenly logged into
many_to_many :rooms, Elemental.TxChat.Room, join_through: Elemental.TxChat.RoomUser, on_delete: :nothing
timestamps()
end
schema "rooms_users" do
belongs_to :room, Elemental.TxChat.Room
belongs_to :user, Elemental.TxChat.User
belongs_to :app, Elemental.TxChat.App
field :status, :integer # offline, online, etc.
timestamps
end
schema "rooms" do
field :name, :string
# The user id that created this room
field :created_by, :integer
field :created_from_app, :integer
many_to_many :users, Elemental.TxChat.User, join_through:
Elemental.TxChat.RoomUser, on_delete: :delete_all
timestamps()
@fields ~w(name created_by created_from_app)
end
How can such an association be saved and retrieved? I came across some examples that do something like user |> change |> put_assoc(:rooms, [room]) |> Repo.update! but the problem here is handling of the intermediate model.
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josevalim
It is likely you don’t want to use many_to_many then because it was designed for cases you don’t really care about the intermediate table. There are a couple different ways you could do this. For example, you could only care about the RoomUser and insert it directly. Assuming you have a room and a user:
If you need to change multiple entries at once, you can rely on Ecto.Changeset and use functions like put_assoc/cast_assoc if you want to.
dotslash
Well, hello Jose! Wasn’t expecting you chime in yourself.
Pardon my ignorance, but are you suggesting that I do away with
many_to_manyand instead treatRoomUseras a separate model? And if I do that, it appears to me that I’ll end up writing separategetandget_bystatements when looking for data in the database. Is that it?Also, you said there are a couple of way to do it. I’m sorry but I can’t find the second in your reply (possibly because I don’t know Ecto that well). Care to point it out, please?
And many thanks in advance!
josevalim
@dotslash I am glad to help!
You can still keep the
many_to_manyaround if you want to. Because you are usingmany_to_manyto read the data, it doesn’t necessarily imply you need to usemany_to_manyto write the data too! For example, if you have both the room and user entries and you want to associate them, doing a single Repo insert with RoomUser may be the simplest approach.However, even if you decide to remove the many_to_many, you can still use preload to load all room_users with the user:
Or use has_many :through to do it at once:
Oops, that was a left-over of the original reply. I will edit it. Sorry about that!
dotslash
Let me just say it’s times like these that I really admire guys like you. I thought I’d get no help on this! Many, many thanks.