Rapid Prototyping with Rails: Lesson 2, Migrations

Items Covered:

1. Changes to Schema
2. data definition statements vs query statements
3. setting up db and creating migration
4. rolling back
   • up/down methods
5. How does Rails keep track of migrations we have run
6. When to modify a migration
7. What happens if I can't run migrations cleanly from end to end

Data Definitions

Data Definitions are to databases what metaprogramming is to code. * statement that alter the database rather than query statements that manipulate the data within a database * the steup commands before we can even query the database * statements that alter the metadata and schema * these occur outside of the code (kinda like setting up the spreadsheet before entering data) * migrations are a good example of this

Migrations

We never want to push our database to other developers when working on a team, we want to push our schema however, and this is done in the form of migrations. - by default, all sqlite3 files under db will be ignored when pushing to git, as per gitignore - To list rake commands pertaining to db:

rake -T | grep db

Database Schema Migrations

• There is table called schema_migrations which stores the migrations of all currently within your database
• rake db:rollback will roll back you most recently ran

Rollbacks

Have you pushed the migration to github... have you shared locally? If not, then rollback is fine, if not.. don't. Because rollback will not create a diff file name. The code changed and its fine on your end.. but for your team members, since the filename hasn't changed, it will cause their application to blow-up since their migration did not re-run. Instead, create a new migration such as rails g migration rename_users. Always google, "rails migrations methods"

What happens if applications can't run cleanly from end-to-end? One of the first thing your team-members will do is run rake db:migrate when cloning your projectand migrations that don't run smoothly is a red flag. Schema.rb - can also be loaded when your migrations don't run from to end-to-end.

=> rake db:schema:load

• you can search for available commands using

=> rake -T | grep schema