Migrations and Active Record Lab
Contents
Section
Objectives
🎃👻🎃
In this lab, you'll be creating the following tables: costumes
, costume_stores
, and haunted_houses
. You'll be creating the following classes: Costume
, CostumeStore
, and HauntedHouse
.
The costumes
table will have four columns:
name
price
size
image url
as well as the two "timestamp" columns: created_at
and updated_at
. This will provide a grand total of six columns.
The costume_stores
table will have seven columns:
name
location
number of costumes, or "costume inventory"
number of employees
whether or not it's still in business
opening time
closing time
The haunted_houses
table will have eight columns:
name
location
theme
price
whether they're family friendly or not
opening date
closing date
long description
Before coding out the creation of these tables, read about Active Record below:
Active Record Review
Active Record is magic. Well, not really. But it does build out a bunch of methods for you. For instance, when it's used properly it will give you access to methods such as create
, save
, and find_by
. Rejoice! Never again will you have to manually build out these methods!
Active Record allows you to create a database that interacts with your class with only a few lines of code. These lines of code go to creating a model, which resides in the app/models
folder, and a migration, which resides in the db/migrate
folder.
The model inherits from ActiveRecord::Base
while the migration inherits from ActiveRecord::Migration
. Many migrations these days have a change
method, but you might also see migrations with an up
and a down
method instead. To use Active Record, you have to stick to some specific naming conventions: while the migrations are plural, the models are singular.
Migrations
When creating migrations, the class names in the migration files must match their file names. For instance, a class in the migration file called 20141013204115_create_candies.rb
must be named CreateCandies
while a class in a migration file called 20130915204319_add_addresses_to_houses.rb
must be called AddAddressesToHouses.
You might notice that in both the examples above, the numbers at the front of the file name were ignored. These numbers are in the form YYYYMMDDHHMMSS
. Later on, these timestamps will become important as Rails uses them to determine which migration should be run and in what order. For instance, if you made a table called dog_walkers
and then added a column to it called rating
, that would be fine as the timestamp on the dog_walkers
table creation would indicate it needs to be migrated before adding the rating
column to it. However, if you did this in reverse order, that is adding a column to a table that doesn't exist, then creating the table, you would get an error.
Migrations, as it was mentioned before, inherit from ActiveRecord::Migration
and usually have a method called change
. In change, you can create a table with the create_table method. This method automatically will create a primary key column called id
, but this default can be overridden if you'd like to customize it.
NOTE: As of Active Record 5.x, we can no longer inherit directly from ActiveRecord::Migration
and must instead specify which version of Active Record / Rails the migration was written for. If we were writing a migration for Active Record 5.1, we would inherit from ActiveRecord::Migration[5.1]
. Don't worry too much about this until you get to the Rails section. Until then, if you encounter an error like this...
...simply add [4.2]
to the end of ActiveRecord::Migration
, exactly as the error message instructs.
Here's a simple example of the create_table
method in action:
The above code would create a table called dogs
with three columns: name
, breed
(both explicitly created), and an implicitly created id
column.
Take a look at a few data types that Active Record supports below:
Data Type
Examples
boolean
true, false
integer
2, -13, 485
string
"Halloween", "Boo!", strings between 1-255 characters
datetime
DateTime.now, DateTime.new(2014,10,31)
float
2.234, 32.2124, -6.342
text
strings between 1 and 2 ^ 32 - 1 characters
Models
Like migrations, models also inherit, but they inherit from ActiveRecord::Base
. A simple model would look like this:
Even though there are no explicit methods for retrieving name
and breed
, this Dog
model is associated with the created dogs
table above. Because of this integration, we can call name
, breed
, and id
on any new instance of the Dog class. For example:
Notice that you had access to reader and writer methods that cooperated with the database that you never had to actually code. You could set the name without ever writing def name=()
and call the self.find_by(attribute)
method without ever teaching your Dog class how to look up data in the database. It's pretty awesome. Take a look at an example below.
Example
Let's say you wanted to make a class called Candy
. Candies should have two attributes, a name (string) and the number of calories (integer), you would write the migration as seen below:
db/migrations/20130915204319_create_candies.rb
Note: You might be wondering what
t.timestamps
is doing here. Well, it creates two new columns,created_at
andupdated_at
. These are handy columns to have around as sometimes you want to query based on the time of creation or update-tion instead of querying using attributes or ids. To read more about timestamps, go to Active Record's docs on them.
While the migration was plural, the model would be singular:
app/models/candy.rb
After saving the code above, running rake db:migrate
will apply the desired changes to the database by running the change method. Then you can alter the database with simple Ruby statements.
For instance, you could create three rows in the table easily:
Retrieving information is just as painless:
As is viewing attributes:
Updating information and viewing table info is also quite simple:
Isn't that amazing? Eleven lines of code allows you to create a table and a class that interact with each other elegantly and efficiently. It builds out methods like, create
, update
, count
, name
, calories
, along with others such as build
and save
.
Instructions
File Structure
You will only be altering code in six files, the three files in the models
folder and the three files in the db/migrations
folder.
Getting Started
This is a test-driven lab so start with the first test and work your way down.
Your models should be no longer than two lines of code.
The first step is to run
bundle install
.Create the Costume class in
app/models/
.Fill out the Active Record migration for costumes such that it passes the specs.
Create the CostumeStore class in
app/models/
.Fill out the Active Record migration for
costume_stores
such that it passes the specs.Create the HauntedHouse class in
app/models/
.Fill out the Active Record migration for haunted_houses such that it passes the specs.
Just like for any other lab, run learn
to view your test progress. However, unlike some of the other labs in thie section, for this lab, when updating an existing migration, you will need to rollback your previous migrations for that table using the rake command rake db:rollback
. Otherwise, the schema will remain unchanged and the changes you make to your migrations will not be seen.
For example, say you've run rake db:migrate
and learn
once to start, and see that you need to add an attribute to the costume_stores
table. Since this table is the second migration of three, you will need to run rake db:rollback
twice to remove the previous migration for this table, then run rake db:migrate
again to update the schema.
Resources
Just look at the code for the example migrations
View Active Record Costume Store on Learn.co and start learning to code for free.
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