Migration Files in Phoenix

Firstly, we all might have or might not have heard about migration files. It's okay let me share you something. For that we should have some basic knowledge regarding migration.

what are the migration files and their purpose?

Migrations are used to modify your database schema over time and to tell the database about expected structure to have.
Phoenix provides many helpers for migrating the database, allowing developers to use Elixir to alter their storage in a way that is database independent.

While working on project I had to create one table in existing database. So I did like as

    PC:~/project/myproject$ mix ecto.gen.migration add_weather_table
    * creating priv/repo/migrations
    * creating priv/repo/migrations/20180331170622_add_weather_table.exs

Here migration file get generated as

    defmodule MyProject.Repo.Migrations.AddWeatherTable do
      use Ecto.Migration
    
      def change do
    
      end
    end

Above we suppose to fill in the empty change/0 function. So I did as

    defmodule MyRepo.Migrations.AddWeatherTable do
      use Ecto.Migration
    
      def change do
        create table("weather") do
          add :city,    :string, size: 40
          add :temp_lo, :integer
          add :temp_hi, :integer
          add :prcp,    :float
    
          timestamps()
        end
      end
    end

After migrating file table get created in database as

    PC:~/project/myproject$ mix ecto.migrate
    
    22:55:43.405 [info]  == Running Trackbees.Repo.Migrations.AddWeatherTable.change/0 forward
    
    22:55:43.423 [info]  == Migrated in 0.0s
Then requirement get changed and need to rolled back that migration to delete that table. So I did using file version number (actually timestamp) as

    mix ecto.rollback -v 20180331170622

Everything worked fine but later I came to know that there is other better way to do all this. We can use up/0 and down/0 functions, where up/0 is used to update your database and down/0 rolls back the requested changes. We can write migration file as

    defmodule MyRepo.Migrations.AddWeatherTable do
      use Ecto.Migration
    
      def up do
        create table("weather") do
          add :city,    :string, size: 40
          add :temp_lo, :integer
          add :temp_hi, :integer
          add :prcp,    :float
    
          timestamps()
        end
      end
    
      def down do
        drop table("weather")
      end
    end

Later in project I implemented it for one other file where I wanted to change the column name for table as below

    defmodule MyRepo.Migrations.RenameStatusIdToMeetingIdInMentions do
      use Ecto.Migration
    
      def up do
        rename table(:mentions), :status_id, to: :meeting_id
      end
    
      def down do
        rename table(:mentions), :meeting_id, to: :status_id
      end
    end