DEV Community: Jędrzej (NJ) Urbański

What you (probably) don't know yet about pattern matching in Ruby?

Jędrzej (NJ) Urbański — Mon, 30 Jan 2023 17:38:36 +0000

So you most probably know about pattern matching in Ruby, right? No...
Well, let's recap what everyone is saying about it.
As for ruby-docs:

Pattern matching is a feature allowing deep matching of structured values: checking the structure and binding the matched parts to local variables.

Yawn... 🥱 Are you sleep? No? Good! Yes? Well, I'm not surprised...
Ok, stay alert now...

This is what typical "Pattern Matching in Ruby 3" article will tell you...

I assume that you already read article like this:

You can pass variable to case statement and match it.

case [1337, 420, 2137]
in foo, *bar
  "matched. foo was: #{foo}"
else
  "not matched."
end
=> "matched. foo was: 1337"

And it's great classic example how you can use pattern matching.

... but are they telling you this?

You can use pattern matching in ruby in other was then just case statement.
Let me introduce you to ✨ => and in ✨ operators, which could be used as standalone expression:

<expression> => <pattern>
<expression> in <pattern>

The only difference between these two is that in returns boolean value, and does not raise an error when pattern is not met. However => raises NoMatchingPatternError or returns nil.

How to use it?

Interesting use case might be when using guard clause. It produces quite powerful ruby one-liner.

result = {mail: "me@nj.test"}
return mail_address if result in {mail: mail_address}

or even shorter:

result = {mail: "me@nj.test"}
return mail if result in {mail:}

Testing with `=>`

Let's create method:

def create_user(params)
    user = {type: :user, name: params[:name]}
    if params.key?(:mail)
      user[:mail] = params[:mail] unless params[:mail].nil?
    end
    user
end

Let's write simple test for it:

# rspec
describe "create_user/1" do
    it "creates user" do
        user = create_user({name: "Mary"})
        user => {type:, name:}
        # => type = :user, name = "Mary"
    end
    it "creates user with mail if given" do
        user = create_user({name: "John", mail: "john@doe.test"})
        user => {type:, name:, mail:}
        # => type = :user, name = "John", mail = "john@doe.test"
    end
end

We are testing here if method create_user/1 are creating has with expected keys.
The test above will pass, because it is matching all key in our user hash.

Now, let's write test that will intentionally fail:

# rspec
describe "create_user/1" do
    # ...
    it "will fail, becuase mail is nil" do
        user = create_user({name: "John", mail: nil})
        user => {type:, name:, mail:}
        # NoMatchingPatternKeyError
    end
end

  1) create_user/1 creates user with mail if given
     Failure/Error: user => {type:, name:, mail:}

     NoMatchingPatternKeyError:
       {:type=>:user, :name=>"John"}: key not found: :mail

As you can see test case above fails, because it cannot match :mail in user-hash. Value for this key does not exists.
We don't need to use asserts, => will throw NoMatchingPatternKeyError which make test fail.

Testing with `in`

Okay, let's now rewrite test using in operator.

# rspec
describe "create_user/1" do
    # ...
    it "will fail, becuase mail is nil" do
        user = create_user({name: "Mary", mail: nil})
        user in {type:, name:, mail:}
        # type = :user, name = "Mary", mail = nil
        assert mail
    end
end

As in does not throw any error, we need to assert mail, so we know it's now nil.

Thanks! 👋 Hope you liked. Comments how you will use pattern matching in your code. 🤗 🗣 ⤵️

How to create unique index skipping empty values in Rails + Postgres?

Jędrzej (NJ) Urbański — Mon, 10 Oct 2022 19:43:26 +0000

Having unique column in rails is easy thing to do. But what if you want to have unique column skipping null values?

Problem

Let's imagine we have application, where invite people to your organisation by sending them invitation code. These should be unique for every organisation, and empty in case we did not generated it yet.

For that let's create organisation model and migration with invitation_code field.

class CreateOrganizations < ActiveRecord::Migration[7.0]
  def change
    create_table :organizations do |t|
      t.string :invitation_code, null: true
    end
  end
end

That's great for start. Now let's create unique index for invitation_code:

add_index :organizations, :invitation_code, unique: true

And add validation in model:

validates :invitation_code, uniqueness: true

Okay, let's see the result:

# for non-empty invitation_code
irb(main):004:0> org = Organization.create(invitation_code: "f00b4r")
=> #<Organization id: "a8dc7fd8-7724-445c-bed4-72c94af99151", invitation_code: "f00b4r">       
irb(main):005:0> org = Organization.create(invitation_code: "f00b4r")
=> #<Organization id: nil, invitation_code: "f00b4r">
irb(main):006:0> org.errors.messages
=> {:invitation_code=>["has already been taken"]}

It's unique for non-empty values, but for empty?

# for empty invitation_code
irb(main):001:0> Organization.create()
=> #<Organization id: nil, invitation_code: nil>
irb(main):002:0> org = Organization.create()
=> #<Organization:0x0000ffff897f2a40 id: nil, invitation_code: nil>
irb(main):003:0> org.errors.messages
=> {:invitation_code=>["has already been taken"]}

Well... as in migration we allow invitation_code to be null, we will have an db error, when creating new record. For that we need to modify our migration.

Solution

We need to modify creating an index as well as model validation.

Let's take a focus on migration first. We need to change just creating index with where statement, so it's unique in scope of non null values.

add_index :organizations, :invitation_code,
  unique: true,
  where: 'invitation_code IS NOT NULL',
  name: 'unique_not_null_invitation_code'

Great, we're all set with db. Now rails model:

validates :invitation_code, uniqueness: { allow_blank: true }

And it all set. 🎉

Test

Let's run migration and quickly test our code in console:

# for empty invitation_code
irb(main):001:0> Organization.create()
=> #<Organization id: "88174146-19c6-40e6-b675-ea04c3e4238f", invitation_code: nil>
irb(main):002:0> Organization.create()
=> #<Organization id: "61d630d1-d244-47e4-af46-880a021a26ca", invitation_code: nil>

Great, two organisation created with null invitation_code. Now try with non-empty code:

# for non-empty invitation_code
irb(main):003:0> Organization.create(invitation_code: "f00b4r")
=> #<Organization id: "8b2ed6e7-76db-4f76-9288-69267ebca5f7", invitation_code: "f00b4r">
irb(main):004:0> org=Organization.create(invitation_code: "f00b4r")
=> #<Organization id: nil, invitation_code: "f00b4r">
irb(main):005:0> org.errors.messages
=> {:invitation_code=>["has already been taken"]}

Works like a charm. Happy hacking! 🧑‍💻