[extra-01] training has a period validity

We move from start/end date to a period date range to represent the
interval where a training take place.

Besides that, new constraints are added to ensure a trainings of a user
can't overlap. These constraints reflect in changeset validation.

priv/repo/migrations/20210627114539_alter_trainings_add_period.exs

@@ -0,0 +1,46 @@
+defmodule Wabanex.Repo.Migrations.AlterTrainingsAddPeriod do
+  use Ecto.Migration
+
+  def up do
+    alter table(:trainings) do
+      add :period, :daterange
+    end
+
+    execute("""
+      update trainings set
+        period=daterange(start_date, end_date, '[)')
+        , updated_at=date_trunc('second', current_timestamp::timestamp)
+    """)
+
+    alter table(:trainings) do
+      remove :start_date
+      remove :end_date
+      modify :period, :daterange, null: false
+    end
+
+    create constraint(:trainings, :non_overlap_period,
+             exclude: "gist (user_id WITH =, period with &&)"
+           )
+  end
+
+  def down do
+    alter table(:trainings) do
+      add :start_date, :date
+      add :end_date, :date
+    end
+
+    execute("""
+      update trainings set
+        start_date=lower(period)
+        , end_date=upper(period)
+        , update_at=date_trunc('second', current_timestamp::timestamp)
+    """)
+
+    drop constraint(:trainings, :non_overlap_period)
+
+    alter table(:trainings) do
+      remove :period
+      modify :start_date, :date, null: false
+    end
+  end
+end