[day-04] add exercise schema

So a training is a collection of exercises that have a description and a
protocol to be followed.

lib/wabanex/exercise.ex

@@ -0,0 +1,30 @@
+defmodule Wabanex.Exercise do
+  use Ecto.Schema
+
+  import Ecto.Changeset
+
+  alias Wabanex.Training
+
+  @primary_key {:id, :binary_id, autogenerate: true}
+  @foreign_key_type :binary_id
+
+  @fields [:name, :video_url, :protocol_description, :repetitions, :training_id]
+  @required_fields [:name, :protocol_description, :repetitions]
+
+  schema "exercises" do
+    field :name, :string
+    field :video_url, :string
+    field :protocol_description, :string
+    field :repetitions, :string
+
+    belongs_to :training, Training
+
+    timestamps()
+  end
+
+  def changeset(%__MODULE__{} = struct, params) do
+    struct
+    |> cast(params, @fields)
+    |> validate_required(@required_fields)
+  end
+end

lib/wabanex/training.ex

@@ -17,6 +17,8 @@ defmodule Wabanex.Training do
 
     belongs_to :user, User
 
+    has_many :exercises, Exercise
+
     timestamps()
   end
 
@@ -24,5 +26,6 @@ defmodule Wabanex.Training do
     %__MODULE__{}
     |> cast(params, @fields)
     |> validate_required(@required_fields)
+    |> cast_assoc(:exercises)
   end
 end