well, so I missed something, because everytime with cast_assoc Ecto tried to insert a new record (ignoring the id field?) and if I forced the id in the cast params of the associated model, it failed because the pk was already in (so I think it was issuing and insert and not an update).
xadhoom
josevalim
Creator of Elixir
When you are sending data without IDs, cast assumes it is new data. If you send data with an ID, Ecto will expect it to exist as a current child in the record, if it doesn’t, it will issue an insert. The only way to get an update is to send an ID and the parent data already has a child with that ID.
1
xadhoom
So, if I have a m2m like posts <> tags I cannot leverage on Ecto to assign existing tags to an existing post, like:
- I have tag_1, tag_2, tag_3
- I have a post, which has (via m2m) tag_1
- I want to update my post in order to associate it with tag_2 and tag_3, removing association (but not record) with tag_1
Basically I’m just updating the join table here.
My update params will have in the :tags field a list of tags, with IDs, (tag_2 and tag_3 in this example)
Since tag_2 and tag_3 are not a current child, Ecto will try to insert them, if I understand correctly?
But they already exists, so kaboom 
So, if I get it correctly, you can update existing child / inserting a new, but changing the relation without touching the existing records must be done “by hand” ?
3
josevalim
Creator of Elixir
Yes, you are 100% correct. And that’s exactly when you use put_assoc/3. put_assoc/3 will just accept the data as is and use it as your children, without trying to do any mapping. I have pushed new docs: Improve docs and tests for cast_assoc/put_assoc · elixir-ecto/ecto@c2c5500 · GitHub
4
OvermindDL1
Ah this is old. ^.^
I’ve not touched many_to_many since my initial tests. Better documentation would be utterly fantastic though. ^.^
1
overallduka
I just found my self in the same situation, i have %{rules: [1,2,3,4]} and needed to convert into %Rule{} array.
1
josevalim
Creator of Elixir
You would do something like:
rules = Repo.all from r in Rule, where: r in ^params["rules"]
Ecto.Changeset.put_assoc(parent, :rules, rules)
I believe we cover similar scenarios in What’s new in Ecto 2.0.
5
krishna_vaguelyright
This whole thread is great. I have broken my head on this for over a week & couldnt get it to work and finally went ahead with the has_many version of it & the frontend team doubting over my skills that I couldn’t get a simple many_to_many work.
I have tried to use cast_assoc, put_assoc everything that I could find online to make it work searched for github repos for examples. Ecto tries to insert the associated enteries everytime!!! with id, structs, changeset, everything…
If somebody could point me to a working example. That would be great! Thanks!
2