leon_cruz
How to validate Ecto Associations properly?
Hello there, I’m getting some problems in validate associations in Ecto. I have two schemas, Video and Playlist (a playlist can have many videos). The Video schema has a following changeset:
def changeset(video, attrs \\ %{}) do
video
|> cast(attrs, [:name, :url, :playlist_id])
|> validate_required([:name, :url, :playlist_id])
end
And Playlist has the following changeset:
def changeset(playlist, attrs \\ %{}) do
playlist
|> cast(attrs, [:name, :category])
|> cast_assoc(:videos, required: true)
|> validate_required([:name, :category])
end
What I want is when create a video, a playlist must be required, and when create a playlist, at least, one video has to be informed. This works for the video case, but when I try to create a playlist, there is a validation error on playlist_id of video. Anyone has some idea how to proceed with this? What I’m doing wrong? Thanks!
Most Liked
soup
Sounds like you’re trying to build everything in one go? But since the playlist doesn’t exist yet, it doesn’t have an id, so it can it be given to the videos, so they fail their validation.
Confession: I can’t remember the last time I used cast_assoc personally, I set association ids manually. I also don’t like how IIRC you have to always give all the relationships in total which can be cumbersome.
I would do something like this, where its a two step transaction. You can also try ecto.multi.
###
### Writing this on my way out the door, so take it as a suggestion
###
def create_playlist_with_videos(name, category, videos) do
# I normally have my own wrapper around transaction, so trying to remember
# exactly without docs.
Repo.transaction(fn ->
with {:ok, playlist} <- create_playlist(name, category),
# now playlist exists with an id so we can create videso
{:ok, videos} <- create_playlist_videos(playlist, videos) do
# transaction() wraps result in {:ok, _}
%{playlist | videos: videos}
else
# can write this nicer, ideally the {:error, _} could just fall
# out and fail the transaction
{:error, e} -> rollback(e)
end
end)
end
defp create_playlist(name, category) do
%Playlist{}
# its ok to have more than one changeset function, i find it useful to
# have at least a create_ and update_ as often they want different
# validations.
|> Playlist.create_changeset(name, category)
|> Repo.insert()
end
defp create_playlist_videos(%Playlist{} = p, videos) do
videos
|> Enum.map(fn v ->
%Video{}
|>Video.create_changeset(playlist.id, v.name, v.url)
|> Repo.insert()
end)
|> Enum.reduce({:ok, []}, fn
# acc is error, perpetuate
# you can use reduce_while to quit out
_, {:error, e} -> {:error, e}
# vid was ok, save and keep going
{:ok, vid}, {:ok, vids} -> {:ok, [vids | vid]}
# vid was error, return error (or collate all errors or ...?)
# how you handle these errors is kind of application specific
{:error, cs} _ -> {:error, :idk_lol}
end)
end
e: you also probably want to put a null: false constraint in the database too if you haven’t.
al2o3cr
cast_assoc combined with validate_required on the child schema’s foreign key will not work (see also a lot of the search results on this forum for cast_assoc). When you’re creating a Playlist, the related Video changesets need to be valid before the Playlist has an ID.
My suggestion to resolve this:
-
use a changeset function in
cast_assoc(:videos, ...)that doesn’t cast or requireplaylist_id; that value will be set by the Ecto machinery automatically -
if a video cannot be created without a
playlist_id, enforce this at the database level with anull: falseconstraint on the column
A rule I find useful to help decide if something should be validated in a changeset function: is there a meaningful way to give feedback about a resulting error to the user? For instance, a blank name or category can show a “can’t be blank” message next to the UI for that value - what feedback could a missing playlist_id give?
leon_cruz
i’ve already setup this
For this case, the message error will be like: “must be select a playlist”
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