Migrations, Databases, Models, and Relationships in Rails

one-to-one:   Username and password
one-to-many:  People and mailing address
              People and pets
              User and instagram photo
many-to-many: Teachers and mod_2 students
              Hashtags and tweets
              
A test environment database can be set up to flush after every run, making it easier to keep track of what's in your db and what you are testing on.
A development environment database usually holds enough data to make sure your app is working as it should. It's usually fake data in the correct format.
-These can be dropped/deleted
A Production database is where actual users' information ends up. NEVER drop this database. It should only be populated with actual user data.

`rails new appname --database=postgresql`or `rails new appname -d postgresql`

`rails g model Model` creates a model for us that is named correctly and has a class within it with the proper inheritance, if you specify field names it creates a migration as well.
It makes it easy to set up the structure of your database.

`rails g model...` creates a model along with a migration file. `rails g migration...` just creates the migration file, and you would need 
to do a `touch app/models/...` to make the model.

`rails g migration RemoveQuantityFromItems quantity:integer`

`students.where(first_name: 'richard')`

`students = Students.find(4)`
`student.update(phone_number: '101-222-3333')` = `student.update(4, phone_number: "92834092134")