Models, Databases, and Relationships

models_databases_and_relationships.markdown

Models, Databases, and Relationships

1. 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?

• A primary key is a unique identifier for records in a database table. A foreign key is a column in a table that references the primary key from a different table. A primary key is usually found in the first column of a database table, and by default is called just 'ID.' A foreign key could be in any column of a table, but is usually an integer referencing the primary key of a different table. The naming convention for a foriegn key is "_id" where is not plural. Foreign keys are used to link data in one table to data in another table.

2. Write down one example of a one-to-one relationship.

• A person has one registered username for a site

3. Write down two examples of a one-to-many relationship.

• I have three pets
• I have many books

4. Write down two examples of a many-to-many relationship.

• A movie has many show times, and the same show time may have many movies
• I have many mentors (and my mentors have many students)

5. What's the difference between test, development, and production databases?

• test: used for tests only, gets cleaned out after every single test
• dev: what you use when building your app, opens in localhost
• production: what is actually deployed to the web and used by real users

6. How do you create a Rails app from the command line with a postgres database?

rails new application_name --database=postgresql

7. What files are created by typing rails g model ...?

• the migration to create the table
• the model file corresponding to the table
• a test file for the new model
• a text fixture (which is a yaml file)

8. What's the difference between typing rails g model ... and rails g migration ...?

• rails g model creates both the model and the migration while rails g migration creates only the migration (and no model)

9. 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?

• To generate the migration rails generate migration RemoveQuantityFromItems quantity:integer

10. Imagine that you have a table students. What is the ActiveRecord query that would return all students with the first name of Richard?

• Student.where(first_name: 'Richard')

11. How would you update the student record with ID 4 to have a new phone number of "101-222-3333"?

• Student.find(4).update(phone_number: '101-222-3333'