Quiz: Lesson 2

Lesson 2 Quiz:

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Quiz: Lesson 1

Quiz: Lesson 1

Quiz from lesson one materials. You can do this yourself, or create a new gist or blog entry, answer the quiz, and post the link to the forums and others can take a look.

1. Why do they call it a relational database?
   A relational database in composed of tables. These tables link together to combine rich datasets. These database tables are created with rows and columns like in a spreadsheet. The rows are where your data is stored and the columns are the schema definition for the table itself. The reason the databases are relational is because tables have a unique primary key column that links to a foreign key column on another table. Thus the tables are relating to one another when writing queries with primary key to foreign key table relationship joins.
2. What is SQL?
   SQL refers to the structured query language for relational databases. This language is standardized across many different database brands. This language allows creating – updating – deleting many different views of datasets in the database.
3. There are two predominant views into a relational database. What are they, and how are they different?
   The two main views in a relational database are the schema and the data. Relational databases are composed of tables which are similar to a spreadsheet as a visualization. The schema is the columns in a table. Each of the columns specifies the data type for that column. The data is in the rows of the table. The data can be inserted, updated, viewed and deleted from one or many rows using the standardized SQL query language.
4. In a table, what do we call the column that serves as the main identifier for a row of data? We’re looking for the general database term, not the column name.
   Primary key.
5. What is a foreign key, and how is it used?In a relational database the foreign key in a table is linked to the primary key of another table in a 1 to many or a many to many table relationship. As an example, the primary key value in say a users table stores a unique id value in a column for that users row. This is usually is an integer and is also a unique value for every row in the users table. In another table say the comments table, can have multiple rows that contain each comment the user creates. To find all the comments that a user creates, the comments table contains a foreign key field that contains the same value of the primary key column of the users table. This is a 1 to many relationship. i.e. each user can have multiple comments using this feature of a relational database.
6. At a high level, describe the ActiveRecord pattern. This has nothing to do with Rails, but the actual pattern that ActiveRecord uses to perform its ORM duties.
   The Active Record Pattern is a pattern that maps:
   • Classes to tables
   • Objects to rows of data within that table
   • Getters/Setters to columns in that table
7. If there’s an ActiveRecord model called “CrazyMonkey”, what should the table name be?

   
   # Use the tableize method on the string "CrazyMonkey"
   
   "CrazyMonkey".tableize
    => "crazy_monkeys"
   
   

   So the table name would be: "crazy_monkeys"

8. If I’m building a 1:M association between Project and Issue, what will the model associations and foreign key be?
   Model Associations:

   
   class Project < ActiveRecord::Base
     has_many :issues
   end
   
   class Issue < ActiveRecord::Base
     belongs_to : project
   end
   
   
   

   The foreign key for this association is “project_id” in the issues table.

9. Given this code

   
   class Zoo < ActiveRecord::Base
     has_many :animals
   end

   • What do you expect the other model to be and what does database schema look like?

          
     
     class Animal < ActiveRecord::Base
       belongs_to :zoo
     end
     
     #migration for the animals table
     
     class CreateAnimals < ActiveRecord::Migration
      def change
        create_table :animals do |t|
          t.string :species
          t.string :name
        end
      end
     end
     
     # ooops
     
     class AddZoo < ActiveRecord::Migration
       def change
         add_column :animals, :zoo_id, :integer
       end
     end
     

   • What are the methods that are now available to a zoo to call related to animals?

     
     zoo = Zoo.create(name: 'San Diego Zoo')
     animal = zoo.animals.create(species: 'Kangaroo')
     animal.name = 'jumpster'
     animal.save
     zoo.animals.name
     zoo.animals
     zoo
     animal = Animal.create(species: 'Alligator', name: 'Buster')
     zoo.animals << animal
     Animal.find_by name: 'jumpster'
     Animal.all
     

   • How do I create an animal called “jumpster” in a zoo called “San Diego Zoo”?

     
     zoo = Zoo.create(name: 'San Diego Zoo')
     animal = zoo.animals.create(species: 'Kangaroo')
     animal.name = 'jumpster'
     animal.save
     

10. What is mass assignment? What’s the non-mass assignment way of setting values?Mass assignment is a way to assign multiple values to attributes via a single assignment operator.

    # Mass assignment example
    
    animal = Animal.create(species: 'Turtle', name: 'Slow Mo')
    
    # Non mass assignment example
    
    
    
    
    animal = Animal.new
    animal.species = 'Turtle'
    animal.name = 'Slow Mo'
    animal.save
    

11. What does this code do? Animal.first
    
    It will return the first database row Animal object in the animals database table.
    
12. If I have a table called “animals” with columns called “name”, and a model called Animal, how do I instantiate an animal object with name set to “Joe”. Which methods makes sure it saves to the database?

    
    animal = Animal.new(name: 'Joe)
    animal.save
    animal = Animal.create(name: 'Joe')
    

13. How does a M:M association work at the database level?
    
    With a Many to Many association the database will have a join table that contains foreign keys of two tables being joined. The foreign keys match the tables being joined, primary keys and creates a M to M relationship
14. What are the two ways to support a M:M association at the ActiveRecord model level? Pros and cons of each approach?
    
    has_and_belongs_to_many
    has_many :through
    has_many :through has a join model and a join table added to this method but allows hanging additional attributes on to the join table itself. has_and_belongs_to_many does not require a join model but requires creating a join table in a manual fashion. You cannot add additional attributes to the join table in this method.
15. Suppose we have a User model and a Group model, and we have a M:M association all set up. How do we associate the two?

    
    class User < ActiveRecord::Base
      has_many :user_groups, foreign_key: :user_id
      has_many :groups, through: :user_groups
    end
    
    class UserGroup < ActiveRecord::Base
      belongs_to :user, foreign_key: :user_id 
      belongs_to :group, foreign_key: :group_id
    end
    
    class Group < ActiveRecord::Base
      has_many :user_groups, foreign_key: :group_id 
      has_many :users, through: :user_groups
    end
    

Active Record

Active Record Models map an object’s attributes to columns in a relational database.

The Active Record Pattern is a pattern that maps:

1. Classes to tables
2. Objects to rows of data within that table
3. Getters/Setters to columns in that table

 

Active Record uses naming conventions for the columns in database tables, depending on the purpose of these columns.

• Foreign keys – These fields should be named following the pattern singularized_table_name_id (e.g., item_id, order_id). These are the fields that Active Record will look for when you create associations between your models.
• Primary keys – By default, Active Record will use an integer column named id as the table’s primary key. When using Active Record Migrations to create your tables, this column will be automatically created.

 

This is how it works.

By default the model looks for a lower case plural name of the model
users database table:

id	username	email
1	joe	joe@yahoo.com
2	bob	bob@apple.com
3	jon	jon@yahoo.com

 

---

user.rb  ActiveRecord model:

# ActiveRecord model

class User < ActiveRecord:Base
 
end

When we instantiate a user object, we automatically have getters and setters for all columns.

Now run these in the rails console …

# Creates an in memory User object:
user = User.new(username: 'bill')

These methods save to the database:
user.save
user = User.create(username: 'rick')

Retrieve column values:
user = User.find_by username: 'joe'
user.email
users = User.where(username: 'joe')

MVC Request Response Diagram