Rspec Matchers

RSpec Matchers

Matchers are the building blocks of RSpec expectations. They define what you're testing and how values should compare.

Basic Syntax

expect(actual).to matcher(expected)
expect(actual).not_to matcher(expected)   # or to_not

Equality Matchers

eq - Value Equality

Tests value equality using ==:

expect(5).to eq(5)
expect("hello").to eq("hello")
expect([1, 2, 3]).to eq([1, 2, 3])

eql - Value and Type Equality

Tests using eql? (same value and type):

expect(5).to eql(5)
expect(5.0).not_to eql(5)  # Different types

equal / be - Identity

Tests object identity using equal?:

a = "hello"
b = a
expect(a).to equal(b)     # Same object
expect(a).to be(b)        # Alias for equal

c = "hello"
expect(a).not_to equal(c) # Different objects, same value

Comparison Matchers

expect(10).to be > 5
expect(10).to be >= 10
expect(10).to be < 20
expect(10).to be <= 10
expect(10).to be_between(5, 15).inclusive
expect(10).to be_between(5, 15).exclusive
expect(10).to be_within(0.1).of(10.05)

Truthiness Matchers

expect(true).to be true
expect(false).to be false
expect(nil).to be nil
expect(nil).to be_nil

expect(1).to be_truthy      # Truthy (not nil/false)
expect(nil).to be_falsey    # Falsey (nil or false)

Type Matchers

expect(user).to be_a(User)
expect(user).to be_an(Admin)        # Alias
expect(user).to be_an_instance_of(User)
expect(user).to be_a_kind_of(User)  # Includes subclasses

Collection Matchers

include

expect([1, 2, 3]).to include(2)
expect([1, 2, 3]).to include(1, 3)
expect("hello world").to include("world")
expect({ a: 1, b: 2 }).to include(a: 1)
expect({ a: 1, b: 2 }).to include(:a)

contain_exactly

Matches array with exact elements, any order:

expect([1, 2, 3]).to contain_exactly(3, 2, 1)
expect([1, 2, 3]).to match_array([3, 1, 2])  # Alias

start_with / end_with

expect([1, 2, 3]).to start_with(1)
expect([1, 2, 3]).to start_with(1, 2)
expect("hello").to start_with("he")
expect("hello").to end_with("lo")

all

Every element matches:

expect([1, 3, 5]).to all(be_odd)
expect(users).to all(be_valid)
expect(numbers).to all(be > 0)

have_attributes

expect(user).to have_attributes(name: "John", age: 30)
expect(user).to have_attributes(
  name: a_string_starting_with("J"),
  age: a_value > 18
)

Predicate Matchers

Any method ending in ? becomes a matcher with be_:

expect(user).to be_valid       # calls user.valid?
expect(list).to be_empty       # calls list.empty?
expect(user).to be_admin       # calls user.admin?
expect(order).to be_pending    # calls order.pending?

For has_* methods, use have_:

expect(hash).to have_key(:name)    # calls hash.has_key?(:name)
expect(user).to have_permissions   # calls user.has_permissions?

String Matchers

expect("hello world").to match(/world/)
expect("hello world").to match("world")
expect("hello").to start_with("he")
expect("hello").to end_with("lo")
expect("hello").to include("ell")

Change Matcher

Test side effects:

expect { user.save }.to change(User, :count).by(1)
expect { user.save }.to change(User, :count).from(0).to(1)
expect { user.activate! }.to change(user, :active).from(false).to(true)

# Block syntax for complex changes
expect { order.complete! }.to change { order.reload.status }.to("completed")

Raise and Throw Matchers

expect { raise "error" }.to raise_error
expect { raise "error" }.to raise_error("error")
expect { raise ArgumentError }.to raise_error(ArgumentError)
expect { raise ArgumentError, "bad arg" }.to raise_error(ArgumentError, "bad arg")
expect { raise ArgumentError, "bad arg" }.to raise_error(ArgumentError, /bad/)

# Throw
expect { throw :done }.to throw_symbol
expect { throw :done }.to throw_symbol(:done)
expect { throw :done, 42 }.to throw_symbol(:done, 42)

Output Matchers

expect { puts "hello" }.to output("hello\n").to_stdout
expect { warn "error" }.to output(/error/).to_stderr
expect { print "hi" }.to output("hi").to_stdout

Compound Matchers

Combine matchers with and / or:

expect(user.age).to be >= 18 and be < 65
expect(string).to start_with("Hello").and end_with("!")
expect(result).to eq(1).or eq(2)

# Using described_as for better failure messages
expect(value).to(be > 0).and(be < 100).described_as("between 0 and 100")

Composable Matchers

Use matchers as arguments to other matchers:

expect(users).to include(
  a_user_with(name: "John"),
  a_user_with(name: "Jane")
)

expect([1, 2, 3]).to include(a_value > 2)
expect(response).to have_attributes(
  status: a_value_between(200, 299),
  body: a_string_including("success")
)

Built-in Aliases

a_string_starting_with("hello")
a_string_ending_with("world")
a_string_including("test")
a_string_matching(/pattern/)
a_value_between(1, 10)
a_value_within(0.1).of(5.0)
a_hash_including(key: value)
an_instance_of(User)
a_kind_of(User)
an_object_eq_to(expected)

Custom Matchers

Simple Custom Matcher

RSpec::Matchers.define :be_a_multiple_of do |expected|
  match do |actual|
    actual % expected == 0
  end
end

expect(9).to be_a_multiple_of(3)

With Failure Messages

RSpec::Matchers.define :be_valid_email do
  match do |actual|
    actual =~ /\A[\w+\-.]+@[a-z\d\-]+(\.[a-z\d\-]+)*\.[a-z]+\z/i
  end

  failure_message do |actual|
    "expected #{actual.inspect} to be a valid email address"
  end

  failure_message_when_negated do |actual|
    "expected #{actual.inspect} not to be a valid email address"
  end
end

With Chain Methods

RSpec::Matchers.define :have_errors_on do |attribute|
  match do |model|
    model.valid?
    @errors = model.errors[attribute]
    @errors.present? && (@message.nil? || @errors.include?(@message))
  end

  chain :with_message do |message|
    @message = message
  end

  failure_message do |model|
    "expected #{model.class} to have errors on #{attribute}"
  end
end

expect(user).to have_errors_on(:email).with_message("is invalid")

Matcher Cheat Sheet

Additional Resources

Reference Files

For advanced patterns:

• references/custom-matchers.md - Complete custom matcher guide
• references/matcher-aliases.md - Full list of matcher aliases

Example Files

• examples/custom_matchers.rb - Working custom matcher examples

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