A protocol that converts data structures into URL parameters.

This protocol is used by URL helpers and other parts of the Phoenix stack. For example, when you write:

user_path(conn, :edit, @user)

Phoenix knows how to extract the :id from @user thanks to this protocol.

By default, Phoenix implements this protocol for integers, binaries, atoms, and structs. For structs, a key :id is assumed, but you may provide a specific implementation.

Nil values cannot be converted to param.

custom-parameters

Custom parameters

In order to customize the parameter for any struct, one can simply implement this protocol.

However, for convenience, this protocol can also be derivable. For example:

defmodule User do
  @derive Phoenix.Param
  defstruct [:id, :username]
end

By default, the derived implementation will also use the :id key. In case the user does not contain an :id key, the key can be specified with an option:

defmodule User do
  @derive {Phoenix.Param, key: :username}
  defstruct [:username]
end

will automatically use :username in URLs.

When using Ecto, you must call @derive before your schema call:

@derive {Phoenix.Param, key: :username}
schema "users" do