A protocol that converts data structures into URL parameters.

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

~p"/user/#{@user}/edit"

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

(Deprecated URL helpers, e.g. user_path(conn, :edit, @user), work the same way.)

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.

The term nil cannot be converted to param.

Custom parameters

In order to customize the parameter for any struct, one can simply implement this protocol. For example for a Date struct:

defimpl Phoenix.Param, for: Date do
  def to_param(date) do
    Date.to_string(date)
  end
end

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