Not sure why this hasn’t been mentioned before (maybe this is a new addition in Ecto), but this seems to be the answer to this issue Ecto.Changeset — Ecto v3.14.0
DragosMocrii
thiagomajesk
I had the same problem recently and this thread was very helpful.
It seems that foreign_key_constraint will only work if you provide the foreign key field while cast_assoc will work if you provide a map - hence @wojtekmach’s solution to validate both cases.
For future reference, a simple use-case to illustrate how it can be done individually might be…
schema "posts" do
field :title, :string
has_many :comments, Comment
end
schema "comments" do
field :text, :string
belongs_to :post, Post
end
Inserting a comment referencing an existing post:
def changeset(comment, attrs) do
comment
|> cast(attrs, [:text])
|> assoc_constraint(:post) # or foreign_key_constraint(:post_id)
|> validate_required([:text])
end
create_comment(%{text: "Nice post!", post_id: 1 })
Inserting a comment with a post all at once:
def changeset(comment, attrs) do
comment
|> cast(attrs, [:text])
|> cast_assoc(:post, required: true)
|> validate_required([:text])
end
create_comment(%{text: "Nice post!", post: %{title: "How to..." } })
1
jeremyjh
This is apples and oranges. foreign_key_constraint just provides a changeset error message when a constraint in the database is violated - which happens if you provide a foreign key value that is not defined in the referenced table. It provides nothing in terms of casting associations, or ensuring that associations are defined.
1
benbonnet
Inserting a comment with a post all at once:
def changeset(comment, attrs) do comment |> cast(attrs, [:text]) |> cast_assoc(:post, required: true) |> validate_required([:text]) end
If I may ask (being quite new to phoenix), using cast_assoc makes the test suuuper slow (about 500ms, compared to some milliseconds without inserting this extra record). We’re inserting two records here instead of one; but still; am i right to doubt such duration is normal ?
0
te_chris
I’ve searched a lot and it still doesn’t seem like there’s a good way to handle the case where you’re creating a child record for a has_many/belongs_to by just passing in the foreign key as an :assoc_id param?
I can’t seem to find anything that actually validates if assoc_id is nil, apart from validates_required…but the docs explicitly say don’t do that. foreign_key_constraint doesn’t do anything if the key is nil/null. cast_assoc expects that you’re modifying/creating the related record, rather just referencing it.
Is the only recommended way to go via the parent?
0