Ecto.Changeset.cast with schemaless types: where are all possible types documented?

I am trying to cast data for which I specify types dynamically at runtime. In my particular case I’d like to validate that a single field is an array of string-keyed maps with single key/value both of which are strings, and the value should be one of a strict set of strings.

It’s about casting a standardised (in our company) sorting parameters passed to our Phoenix controllers.

The Phoenix parameters format goes like this:

  sort: [ %{"birthdate" => "desc"}, %{"inserted_at" => "asc"} ]
  # all other parameters

(This is achieved by sending an HTTP query like so: ?sort[][birthdate]=desc&sort[][inserted_at]=asc)

At the moment I am doing this:

fields = [:sort]
params = %{"sort" => [%{"birthdate" => "desc"}, %{"inserted_at" => "desc"}]}
types = %{sort: {:array, :map}}
Ecto.Changeset.cast({%{}, types}, params, fields)

And it works (meaning the resulting changeset is valid).

However, I wonder if there’s a way to achieve finer-grained control of the type of the sort key. I blindly tried this: types = %{sort: {:array, {:map, :string}}} and it also worked, but to be completely honest I have no idea why. :003:

Is there a way to achieve this finer-grained type control when casting with Ecto’s Changeset? If so, where is it documented?

The docs start here:
Wherever you read inner_type you can nest any other type available, which is what makes {:array, {:map, :string}} possible.


I’d also not really make the casting to strict, because all you get as error would be is invalid. Rather make casting ensure you get a known format of data and let other validation steps validate the details with proper error messages.

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Thank you. But what does it represent really? A map with string keys? With string values? Or both?

Really good advice, appreciate it. How would you go about doing it?

As keys are always in “danger” of being serialized to strings (because they’re external to the VM when stored in a db) it means {:map, :string} is a map of :string values.


I ended up eschewing cast for some of those special fields altogether. I ignore them in the initial cast call and then call a function that does exhaustive type and data shape validation, calling add_error or put_change wherever appropriate. Communicates errors and intent much better than having a generic "is invalid" message indeed.