add difference operator to the cheat sheet
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tree: 58238c9197d3be74ee27b4e48871949517d67d0a
  1. support/
  2. tests/
  3. Gemfile
  4. Gemfile.lock

ActiveRecord and Arrays (and Hashesh)

Now that we've started our projects we're going to take a look at some of the capabilities that ActiveRecord has to offer us in addition to manipulating Array's (a very valuable skill for any Rubyist). We're going to focus heavily on documentation and how to look stuff up.


Fork and clone the repo to your local machine. Navigate to this directory and run:

$ bundle install

Then follow the directions in this readme.

1) Ruby Docs && Arrays

For the first part of this exercise we'll be doing something a bit different. Rather than try to cram everything we need to learn into a Rails project we're going to do some Ruby exercises. In addition to learning about arrays, we'll also learn a little about testing. In a programming language as flexible and dynamic as Ruby, writing tests are a good way to know that your logic is valid. You make assertions and when you run the tests, you find out if your assertions were correct or not.

There are many different testing frameworks and many styles of testing. You may hear of TDD (test driven development). For now we are going to use 'tests/unit' which comes with Ruby. If you open the file tests/array_test you'll see our test class ArrayTest. It inherits from Test::Unit::TestCase. We define tests by making new methods with distinct method names that start with test_. Here I've written a number of tests, it is your job to get them passing. In programming when two people code together and one writes the tests and the other gets them to pass, this is called ping pong pairing! Though usually they swap between tests.

Note: an error is a type of failure, so failure count = errors + failures

First, run the test file by navigating to this directory in your terminal and then running the test file with ruby

$ ruby tests/array_test.rb

You should see a number of failing tests go by, at the bottom you will get a recap of which tests passed and which failed like this:

Finished tests in 0.000928s, 1077.5862 tests/s, 1077.5862 assertions/s.

1 tests, 1 assertions, 0 failures, 0 errors, 0 skips

Your numbers will be different. Once you've confirmed that we have failing tests we'll need to make them passing.

In the output you should see a line that looks like this:

test_true(ArrayTest) [tests/array_test.rb:6]:
Failed assertion, no message given.

Here it is telling us that in the class ArrayTest which lives in the file tests/array_test.rb we got a failure in a method called test_true. Open up tests/array_test.rb and find that method. It should look like this:

def test_true
  assert false

You can fix this test by changing the false to true. Do this and run the tests again. You should now see one less failing test.

Before we go any further find the ruby documentation on Array, use Google if you have to. Once you've found the Array page find the Array#reject method. This is an alias to Array#delete_if (though I like the semantics or reject better), since the method is aliased we must find the Array#delete_if documentation. Once you've found it, copy the link and enter it into Question # 1 & 2 in

Link to the Array documentation Direct Link to the Array#delete_if method

If you're having a hard time finding it, use the navigation on the left of the page. Your link should contain an anchor character # and if you paste it into chrome, you should go straight to the delete_if method.

Now take a look at the example from the docs, you should see something like this:

a = [ "a", "b", "c" ]
a.delete_if {|x| x >= "b" }   #=> ["a"]

Here we are setting up a junk array to variable a with the values "a", "b", and "c" we then call the delete_if method on that array and delete if the element is greater than or equal to "b". This might seem unusual, but consider in computer programming all characters are mapped to integers. All the characters that match that criteria are removed from the array, this leaves us with only "a".

If you look at the docs again you'll notice this line:

delete_if {|item| block } → ary

This is short hand to show us one way of calling the delete_if method. In this short hand, we see we can pass our method a block, and it will return us an Array. This is why in the above example the result was ["a"] and not simply "a". This form of the method will always return an array.

Feel free to open up IRB and play with this method. While I'm developing I often have a session open (or rails console if I'm working with Rails). When I find a method I haven't used before I copy and paste the example into IRB and make sure it runs, then I try to make up my own example and make sure I get a response that makes sense to me. For delete_if I might try something like this:

a = [1,2,3,4,5]
a.delete_if {|item| item == 2 || item == 5 }

Before I run it I make a mental note of what I think the outcome should be, in this case we are deleting if the element in the array is 2 or if it is 5 so I would expect [1,3,4] to be the output. Then I would go ahead and run it:

$ irb
> a = [1,2,3,4,5]
> a.delete_if {|item| item == 2 || item == 5 }
=> [1, 3, 4]

Perfect, worked just like we thought it would. Since reject is an alias of delete_if, lets try replacing delete_if with reject and see if we get the same outcome:

a = [1,2,3,4,5]
a.reject {|item| item == 2 || item == 5 }
# => [1, 3, 4]

Awesome. Now that we've got a handle on reject lets fix a test together. Open up your tests/array_test.rb and find a test called test_reject. Here I've already written a failing test that looks like this:

def test_reject
  array = [1,2,3,4,5,6,7,8]
  # put your code here
  assert_equal [1,3,5,7], array

I want you to use the Array#reject method to make the test pass without changing the first or last line. Let's do this one together.

In programming there are seemingly unlimited ways to complete a given task. As you learn more, you'll be better equipped to pick out the right tools for the right jobs. Just as a master carpenter wouldn't pick a screwdriver to hammer in a nail you will get an instinct for how to perform most programming tasks. For now we're more focused on getting the nail in, we'll worry about how it got there later. For now let's think about how we could get these two arrays to equal one another with the reject method.

First of all, reject takes out it helps that [1,3,5,7] are all elements in the variable array. If we look at the difference between the two you'll notice we're missing [2,4,6,8] so while I'm pretty sure we could get away writing something like this:

array = array.reject {|x| x == 2 || x == 4 || x == 6 || x == 8}

It's not super easy to read and I'll bet there is some way to see if a number is even with ruby (which 2,4,6 and 8 are). So lets take a look at the docs for our numbers. In IRB let's get the class of our number:

$ irb
> 8.class
=> Fixnum

So we're using the Fixnum class. Find the Fixnum documentation online. The easiest way to find it is with google. The second easiest is to navigate to the Core section of the ruby docs and find Fixnum.

Enter the url to the fixnum documentation down in

If you've done computer science before you might be expecting this next section to talk about the modulus operator %, if you want to use that you can. Note that the docs for it point at Numberic.divmod and then when you finally find the right method the example is a bit cryptic. Some methods are documented better than others. You'll also find that the docs aren't always the best way to discover methods, but they're crucial for understanding them.

For now let's go back to Fixnum docs and scan the available methods on the left side. You'll see that there are a number of methods such as + and - which don't look like methods at all (though in Ruby they are!). If you keep reading until you hit the methods that begin with "e" you'll see we have a method called even?. Click on it.

Enter the url to the even? operator into`

To use this in our test, we could simply reject all the even elements. Open tests/array_test.rb and find the test_reject method again, add this code between the two existing lines so it looks like this:

def test_reject
  array = [1,2,3,4,5,6,7,8]
  array = array.reject {|x| x.even? }
  assert_equal [1,3,5,7], array

Now run your tests again, you should see one less failure! Great job! You just made your first test pass! If it isn't passing, look at the test output for test_reject and try again.

There are several other ways to make this test pass for example using the odd? method with the bang operator (!) which will reverse the logic:

def test_reject
  array = [1,2,3,4,5,6,7,8]
  array = array.reject {|x| !x.odd? }
  assert_equal [1,3,5,7], array

Or we could even use the modulus operator we hinted at previously:

def test_reject
  array = [1,2,3,4,5,6,7,8]
  array = array.reject {|x| x%2 == 0 }
  assert_equal [1,3,5,7], array

At the end of the day, how you get the nail in doesn't matter. Pick the solution that is the most readable and understandable to you. Sometimes when I'm working on a project and can't find an easy way to manipulate something (like an array, string, hash, etc.) I'll flip through the available methods and see if something pops out that I hadn't seen before. Sometimes though, I know exactly what I want to do such as find an "even fixnum in ruby" and I'll search for "even fixnum ruby" in Google. I often find that the docs with an answer will be on the first page of results.

Hopefully by now you shouldn't feel completely intimidated by the docs. Answer a few more questions and then it will be time to set you loose on fixing all those test failures.

Link to Hash docs:

Direct Link to Hash.keys docs:

Link to String docs:

Direct Link to String#upcase docs:

Head back to the Array docs and then you'll start fixing the rest of the tests. In addition to assert and assert_equal there are a number of different assertion statements we can use. Since our test class inherits from Test::Unit::TestCase we can look at those docs. From there you'll see that we include a module called Test::Unit::Assertions here you'll find quite a few assertions we could have used

Go through the rest of the methods in the tests/array_test.rb. Switch between writing code and looking up docs. Once you get a test to pass, move on to the next until all are passing. There are comments above each method that should help you complete your task.

When all the tests are passing, commit your questions and the fixed tests to git.

If you think you're being super smart and decide look at other people's pull requests, you're only hurting yourself. If you get stuck use Google or StackOverflow. Feel free to work with others, but make sure you understand how to get all of the answers by yourself. After you're done with your assignment you can look at other's work and see if they did something different from you. Maybe you'll pick up a trick or two.

If you get stuck you might want to look at the documentation for these methods.


#2) ActiveRecord

Okay, you've survived the onslaught of Array questions. Hopefully you've got some awesome array methods under your belt and you're ready to tackle ActiveRecord. Like we did last time we're going to be using tests.

If seeing a bunch of failures bothers you while you're coding you can go through and add skip to the first line of tests, making sure to remove them as you're working on getting that test fixed.

We will be using ActiveRecord without Rails in this exercise. Because of that you won't have the usual tools like $ rails console to play around with queries. Open up tests/active_record_test.rb.

In the first few lines you'll see that we're requiring a relative file active_record_support.rb this code will create an in memory SQLite database and using ActiveRecord.

If you want to use this in your irb you can copy the contents of support/active_record_support.rb and paste it into irb. You can then create Dealerships and Cars as you would with Rails. To help you out you can also copy the code in support/fake_data.rb and paste it into the same irb session. This will create a number of fake dealerships and cars for you to run SQL queries on.

In the last section we looked for ruby documentation. In this section we'll be using ActiveRecord which is the ORM that Rails uses. Since it does not come with Ruby, you will have to find the ActiveRecord documentation, some of it is included in Rails documentation and some of it is stand alone. There are two main sources for rails documentation, one is method documentation like we saw with ruby and the other is a set of Guides. The guides are structured in more of a storytelling manner and more focused on examples than technical implementation. First off open a new tab and visit then find the link to conditions on the right and click on Conditions add this to your questions file:

Direct Link to Rail's ActiveRecord querying conditions

Here we see much of what was discussed in class, how to avoid SQL injection and how to use some common query methods. The Rails Guides are by far my favorite resource when working with Rails code. This page is both comprehensive and understandable. If you're currious about a specific active record condition this would be a great place to start.

The second place you'll find docs is on Visit and on the left side you can search for documentation based on class name. Here we're using ActiveRecord::Base so you can select ActiveRecord and then Base. You should now see something that looks like This documentation is much closer in style to the Ruby documentation. It is mostly auto generated through a system called rdoc that allows programmers to embed documentation inside of comments along side of their code. There are other programs available (I personally prefer YARD to document code). The quality of this documentation can vary greatly between topic, class, and method. Sometimes I prefer to use APIdock to browse the Rails docs. It is the same documentation, but I prefer the layout. Go ahead and find the apidock link to ActiveRecord::Base, add this to your questions:

Link from APIdock to Rail's ActiveRecord querying conditions

While we're linking things add a link to the Rails guides to your question file.

Link to Rails guides

Now that we've got our docs all ready we're set to fix some tests.


$ ruby tests/active_record_test.rb

You should see some tables being created:

-- create_table(:dealerships)
   -> 0.0091s
-- create_table(:cars)
   -> 0.0008s

Don't worry this is only "in memory" or in RAM. As soon as we're done running our tests our database will be gone, so we can keep running the same tests. Because of this we can add data to our database every time it is run to perform our tests, and they will be cleaned up at the end. I've already added data, but now it's your job to pull it out.

Lets take a look at test_regular_where in your tests file. It should look something like this:

def test_regular_where
  name =
  d    = Dealership.create(:name => name)
  # put your code here
  assert_equal d, dealership

Here we're using a library called Faker to provide us with human readable fake names. First we assign a name variable, then add a dealership to the library. Now it's our job to pull the same dealership from the database and assign it to a variable of dealership we can do that using a where query and our hash syntax. Since we don't know what the random name variable is, we'll need to use it again in our query. This will make the tests pass:

def test_regular_where
  name =
  d    = Dealership.create(:name => name)
  dealership = Dealership.where(:name => name).first
  assert_equal d, dealership

We could have also used the string substitution method (?).

dealership =  Dealership.where("name = (?)", name).where

By now you should be fairly used to finding records like this. Run the tests to make sure you got that one passing. Continue on with the rest of the test until you get all of them passing.

When all the test pass commit to git and push to github.


Get the tests in tests/hash_test.rb to pass.

When all the test pass commit to git and push to github.


That's it. You got your first taste of tests, and got a somewhat forced introduction to documentation. You also got more exposure to Arrays and ActiveRecord.

Programming is kinda complicated, so the easier you can make it on yourself with docs and google the quicker you will improve. While they're helpful, they're not magic bullets. Spending a few minutes browsing documentation can save you hours of hair pulling. Not that you have to read word for word every bit of docs on every page you see, but you'll certainly develop an appreciation for critical reading.

Hopefully these doc and searching skills will help you with your projects. As a Ruby programmer especially if you're working with Rails, the the docs on Arrays and ActiveRecord are invaluable resources. If you feel like you didn't really learn anything, and that you cheated your way through by looking everything up with docs that's okay. You're not expected to keep every method definition of very class in your head. In fact some of the best programmers I know constantly check what they think they know with docs.

Great job, and don't forget to push your code back up to github.