Yeah hard to believe there isn’t a single-changeset solution for this scenario.
wfgilman
wojtekmach
Hex Core Team
it’s pretty hacky, but perhaps this would work for you?
defmodule Rating do
# ...
def changeset(rating, params \\ %{}) do
cast(rating, params, ~w(rating place_id))
|> validate_required(~w(rating)a)
|> cast_or_constraint_assoc(:place)
end
defp cast_or_constraint_assoc(changeset, name) do
{:assoc, %{owner_key: key}} = changeset.types[name]
if changeset.changes[key] do
assoc_constraint(changeset, name)
else
cast_assoc(changeset, name, required: true)
end
end
end
I’d stick to exposing two different changeset functions though (and probably have a 3rd private changeset function that has common stuff)
7
dotdotdotPaul
Nice solution for keeping the shared validations all in one place. Was just hoping there was a better “best practice”. 
0
josevalim
Creator of Elixir
dotdotdotPaul
It does, but then you can’t use the changeset by simply passing in the foreign key IDs (in this example, you couldn’t create a Rating.changeset(%Rating{}, %{rating: 5, place_id: 42}) (because this causes an error during the cast_assoc, since :place isn’t being used).
0
josevalim
Creator of Elixir
axelson
Scenic Core Team
Even if we’re using wojtekmach’s solution aren’t we still missing a validation that some version of place as passed in? Either as place or place_id?
If we do want that validation along with the constraint do we have any options?
0
jeremyjh
It does do that - if it is passed in as place_id it is validated with assoc_constraint; otherwise it is checked with cast_assoc required: true.
However the code as listed is really not complete because it does not handle updates to the record that do not include a change to the place; e.g. record is loaded from the db, place_id is set, but because place_id is not changed cast_assoc fails on it. In my project I’m using a slightly modified version that will pass the changeset if it contains the key (place_id) already:
def cast_or_constraint_assoc(changeset, name) do
{:assoc, %{owner_key: key}} = changeset.types[name]
#assoc id was directly set? confirm its valid
if changeset.changes[key] do
assoc_constraint(changeset, name)
else
#assoc key is already present, and not changed? do nothing
if Map.get(changeset.data, key) do
changeset
else
#we need to insert a new assoc (or errors)
cast_assoc(changeset, name, required: true)
end
end
end
4
jeremyjh
The second part of this comment, and the code was wrong…I don’t have that original project in front of me but I’ve since tried to use this code in another project and it does not work because the association doesn’t show up in the changes collection when it is just passed in update params as a map. I now think @wojtekmach original code is the best solution and I suspect the problem I had in the previous project was because I did not preload the association I was updating, or maybe I passed the name of database field rather than the name of the association, e.g. in this example it should be invoked in the changeset as |> cast_or_constraint_assoc(:place)
To save scrolling, this is the code I think is best:
defp cast_or_constraint_assoc(changeset, name) do
{:assoc, %{owner_key: key}} = changeset.types[name]
if changeset.changes[key] do
assoc_constraint(changeset, name)
else
cast_assoc(changeset, name, required: true)
end
end
1
ndac_todoroki
For people looking for more validation, the code below works pretty well for me.
Using this code helps when you want to deal with a new Map or an existing Struct or an existing id was given for the association…
def put_or_cast_or_constraint_assoc(changeset, name) do
{:assoc, %{owner_key: key, related: type}} = changeset.types[name] |> IO.inspect
if changeset.changes[key] do
assoc_constraint(changeset, name)
else
case val = changeset.params[name |> Atom.to_string] do
%correct_type{} when correct_type == type ->
put_assoc(changeset, name, val)
%wrong_type{} ->
add_error(changeset, name, "Wrong struct given to #{key}", given: wrong_type, wanted: type)
%{} ->
cast_assoc(changeset, name, required: true)
_ ->
add_error(changeset, name, "No valid #{name} nor #{key}")
end
end
end
2