Rails questions

gistfile1.md

Answers:

1. Because data sets can be related through the use of foreign keys

2. Structrured Query Language, it is a stnadard language to query database to retrieve / store data

3. Schema aka Strucre view which shows the list of the column names, data view whcih shows the actual data (rows of data)

4. The Primary key, which is usually the id

5. The forgeing key is used in rows in a table to join other rows from another table, where the foreign key is the primary key

6. About ActiveRecord pattern we can say that each table is represented by a Class and each row (of that table) by an instance of that Class. The Class usually also provides attr accessor methods to retrieve / store data in each column

7. crazy_monkeys - we can also use the very useful 'CrazyMonkey'.tableize method which returns => "crazy_monkeys"

8. Assuming the following: Project has_many issues and Issue belongs_to project than the foreign_key will be on the issues table in the db. As a simple rule the foreign_key is usually on the belongs_to model side.

9. .class Animal < ActiveRecord::Base belongs_to :zoo end . Schema of the table animals: . id (integer, created by default) . name (string, manually added with the mgration file) . zoo_id (integer, manually added with the migration) . Some of the methods available to the Zoo class are: . zoo = Zoo.first . zoo.animals (returns an array like structure, of all animals objects belonging to zoo) . zoo.animals << (shovel operator appends an oobject to the array) . zoo.animals.delete(animal id) (same as destroy) . zoo.animals.ids (returns array of ids of all animals assiciated to zoo) . zoo.animals.clear (set the foreign key of animals associated to zoo to NULL) . zoo.animals.find(animal id) . zoo.animals.build (instantiates a new animal but does not save to db, create does the same but saves to db) . as follows: . zoo = Zoo.find_by(name: "San Diego Zoo") assuming there is a column called :name which holds the name of the zoo . zoo.animals.create(name: "jumpster")

10. Mass assignment is a way to create a instance of a ActiveRecord class by assigning all its attr_accessor in with one single line of code as in Animal.new(name: "zebra", foreign_key: "1") . To assign without mass assignment Animal.first.name=("zebra")

11. returns the first animal object from all animal instances of class Animal

12. Animal.create(name: "Joe")

13. a M:M association main aspect at a db level is that it needs a join table that has foreign keys of both sides

14. Has Many and Belongs to Many vs Has Many Through. The main difference is that HMT allows to reference the join table from rails becuase it has a Model for the join table, whereas HMBTM has the join table in the db but does not allow to reference it directly in rails

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? At a Model level we would need to define the following 3 Models:

class User < ActiveRecord::Base
   has_many :user_groups
   has_many :groups, through: :user_groups
end

class Group < ActiveRecord::Base
   has_many :user_groups
   has_many :users, through: :user_groups
end

class UserGroup < ActiveRecord::Base
   belongs_to :user
   belongs_to :group
end

We would finally need 3 different tables at a db level:

users
groups
user_groups (this is the join table, it has 2 foreign keys that refer to the users table and the groups table)