Add migration

priv/repo/migrations/20191026191753_set_not_null_for_thread_mutes.exs

+defmodule Pleroma.Repo.Migrations.SetNotNullForThreadMutes do
+  use Ecto.Migration
+
+  # modify/3 function will require index recreation, so using execute/1 instead
+
+  def up do
+    execute("ALTER TABLE thread_mutes
+    ALTER COLUMN user_id SET NOT NULL,
+    ALTER COLUMN context SET NOT NULL")
+  end
+
+  def down do
+    execute("ALTER TABLE thread_mutes
+    ALTER COLUMN user_id DROP NOT NULL,
+    ALTER COLUMN context DROP NOT NULL")
+  end
+end