Method: ActiveSupport::Testing::Assertions#assert_difference

Defined in:
activesupport/lib/active_support/testing/assertions.rb

#assert_difference(expression, *args, &block) ⇒ Object

Test numeric difference between the return value of an expression as a result of what is evaluated in the yielded block.

assert_difference 'Article.count' do
  post :create, params: { article: {...} }
end

An arbitrary expression is passed in and evaluated.

assert_difference 'Article.last.comments(:reload).size' do
  post :create, params: { comment: {...} }
end

An arbitrary positive or negative difference can be specified. The default is 1.

assert_difference 'Article.count', -1 do
  post :delete, params: { id: ... }
end

An array of expressions can also be passed in and evaluated.

assert_difference [ 'Article.count', 'Post.count' ], 2 do
  post :create, params: { article: {...} }
end

A hash of expressions/numeric differences can also be passed in and evaluated.

assert_difference ->{ Article.count } => 1, ->{ Notification.count } => 2 do
  post :create, params: { article: {...} }
end

A lambda or a list of lambdas can be passed in and evaluated:

assert_difference ->{ Article.count }, 2 do
  post :create, params: { article: {...} }
end

assert_difference [->{ Article.count }, ->{ Post.count }], 2 do
  post :create, params: { article: {...} }
end

An error message can be specified.

assert_difference 'Article.count', -1, 'An Article should be destroyed' do
  post :delete, params: { id: ... }
end



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# File 'activesupport/lib/active_support/testing/assertions.rb', line 101

def assert_difference(expression, *args, &block)
  expressions =
    if expression.is_a?(Hash)
      message = args[0]
      expression
    else
      difference = args[0] || 1
      message = args[1]
      Array(expression).index_with(difference)
    end

  exps = expressions.keys.map { |e|
    e.respond_to?(:call) ? e : lambda { eval(e, block.binding) }
  }
  before = exps.map(&:call)

  retval = _assert_nothing_raised_or_warn("assert_difference", &block)

  expressions.zip(exps, before) do |(code, diff), exp, before_value|
    actual = exp.call
    rich_message = -> do
      code_string = code.respond_to?(:call) ? _callable_to_source_string(code) : code
      error = "`#{code_string}` didn't change by #{diff}, but by #{actual - before_value}"
      error = "#{message}.\n#{error}" if message
      error
    end
    assert_equal(before_value + diff, actual, rich_message)
  end

  retval
end