Elixir map check if not empty and key exists

Question

I am trying to figure out a way to check if the params hash in a Phoenix app (using Elixir) has a particular key or not.

In the below changeset function in a model, the params is defaulted to :empty.

def changeset(model, params \\ :empty) do

I need to figure out if a key named :username exists in the hash.

Solution 1

Just a small terminology thing, params is a map and not a hash. This is relevant when knowing where to look in the documentation.

For a map there is has_key?/2 which returns true or false.

Map.has_key?(params, :name)

Since you are using an Ecto changeset then you can also use Ecto.Changeset.get_change/3.

get_change(changeset, key, default \\ nil)

This returns default if key is not set. Please note that if key is set to nil then nil will still be returned. If nil is a permitted value for your change then you may want to set a different default parameter.

Solution 2

Gazeler's answer is obviously really good. I would only add pattern matching to the mix, as it seems to me, that's the clearest solution, that works not only with phoenix, but with any map anywhere in Elixir.

# head-only declaration for default params
def changeset(model, params \\ :empty)

def changeset(model, %{"username" => _} = params) do
  # username field is in params
end

def changeset(model, params) do
  # username is not in params
end

Comments

[Puzirki]

Are you sure is it correct? map = %{"username" => "Name", "ONE_MORE_KEY" => "VALUE"} changeset(map) ----- # username is not in params

[Jeremy Cochoy]

@Puzirki It may looks conter intuitive at first look but any map with the key "username" will match the pattern %{"username" => _}. You can test it in the iex console by writing %{"username" => val} = %{"x" => 0, "y" => 1, "username" => 10}.