ModelsDatabasesRelationships.md

## Models, Databases, Relationships in Rails

#### What is the difference between a primary key and a foreign key? Where would we find a primary key? What would it be called by default? Where would we find a foreign key? What is the naming convention for a foreign key?
  Primary key is a unique identifier in Tables. It can exist as a foreign key in AnotherTables or other tables with which it has a relationship. "id" - default name. "table_id" in AnotherTables.

#### Write down one example of:
* a `one-to-one `relationship: (current democratic society enables) one country has one president
* a `one-to-many relationship': one country has many citizens
* a `many-to-many relationship`: (given current industrial agricultural norms...I assume...) each agricultural crop has many producers, each producer has many agricultural crops

#### What's the difference between test, development, and production databases?
* Test database holds data utilized when the environment is testing. Data is loaded in. Its purpose is to enable the development team to test functionality with fake data.
* Development database holds data that is created and updated, etc. when the environment is in development. This means that there are people interacting with application but still in a controlled sense. 
* Production database create, update, etc. data when the application is live and so there may be a combo of indirect and direct impact on the data from an external user utilizing the application. "Sacred" database. Do not delete production data.
* Each environment represents different states of an application.

#### How do you create a Rails app from the command line with a postgres database?
 ``` rails new [app_name] --database=postgresql ```
 ``` rails new [app_name] --d=postsgresql ```
 
#### What files are created by typing rails g model ...?
* rails g model ... == rails generate model ...
* files created: 2 test files (fixture, test), model, migration file

#### What's the difference between typing rails g model ... and rails g migration ...?
* rails g model generates four files related to the model specified, which are two test files (fixtures, test), model file, and a migration file 
* rails g migration performs ActiveRecord migration actions, which include creating migration files and updating the schema.

#### Imagine that the items table has a category called quantity. What command would you type if you wanted to get rid of the quantity attribute?
* rails g migration RemoveQuantityColumninItems quantity:integer