Put constraint on PhoenixFramework changeset - phoenix-framework

Question

I have three models in Phoenix Framework, User, Post, Vote. Users can down vote the post only when they have more than 1 point. The user point is calculated by the up votes their posts got from other users.
This is what I defined in my Vote model:
schema "votes" do
field :type, :integer # -1 0 1
belongs_to :user, News.User
belongs_to :post, News.Post
timestamps()
end
Because the user points is not defined in Vote schema, so I can't use validate_change or add_error directly in model, unless I read other models' data to decide whether to add error to changeset, obviously it will be doing too many things in Vote model.
Where should I put the constraint? Controller or model?
Maybe I should place constraint on the database, make sure the user point never get below zero? I found something like trigger. But how will PostgreSQL trigger return their result to the changeset?
Updated (this one works, but I'm not sure if it's the best way)
I tried it in my controller:
def create(conn, %{"vote" => vote_params}, user) do
changeset = user
|> build_assoc(:votes)
|> Vote.changeset(vote_params)
changeset = if user.point < 1 do
Ecto.Changeset.add_error(changeset, :user_id, "You points is not enough.")
end
case Repo.insert(changeset) do
{:ok, vote} ->
conn
|> put_status(:created)
|> put_resp_header("location", vote_path(conn, :show, vote))
|> render("show.json", vote: vote)
{:error, changeset} ->
conn
|> put_status(:unprocessable_entity)
|> render(WechatNews.ChangesetView, "error.json", changeset: changeset)
end
end
It's easy, but I have to repeat it in the update action too.

Answer 1

You could do something along the lines of
def changeset(struct, params) do
struct
|> cast([:points], params)
|> validate_required(:points)
end
def point_changeset(struct, params) do
struct
|> changeset(params)
|> check_points
end
defp check_points(changeset) do
if get_field(changeset, :points) < 1 do
add_error(changeset, :points, "too low")
else
changeset
end
end
This would give you a function point_changeset/2, which you can use only when you need to check that a user can do a certain action based on their points. It still calls the main changeset/2 function that has your validations that you want to always run.