Method: ActiveSupport::Testing::Assertions#assert_changes

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

#assert_changes(expression, message = nil, from: UNTRACKED, to: UNTRACKED, &block) ⇒ Object

Assertion that the result of evaluating an expression is changed before and after invoking the passed in block.

assert_changes 'Status.all_good?' do
  post :create, params: { status: { ok: false } }
end

You can pass the block as a string to be evaluated in the context of the block. A lambda can be passed for the block as well.

assert_changes -> { Status.all_good? } do
  post :create, params: { status: { ok: false } }
end

The assertion is useful to test side effects. The passed block can be anything that can be converted to string with #to_s.

assert_changes :@object do
  @object = 42
end

The keyword arguments :from and :to can be given to specify the expected initial value and the expected value after the block was executed.

assert_changes :@object, from: nil, to: :foo do
  @object = :foo
end

An error message can be specified.

assert_changes -> { Status.all_good? }, 'Expected the status to be bad' do
  post :create, params: { status: { incident: true } }
end



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# File 'activesupport/lib/active_support/testing/assertions.rb', line 195

def assert_changes(expression, message = nil, from: UNTRACKED, to: UNTRACKED, &block)
  exp = expression.respond_to?(:call) ? expression : -> { eval(expression.to_s, block.binding) }

  before = exp.call
  retval = _assert_nothing_raised_or_warn("assert_changes", &block)

  unless from == UNTRACKED
    rich_message = -> do
      error = "Expected change from #{from.inspect}, got #{before.inspect}"
      error = "#{message}.\n#{error}" if message
      error
    end
    assert from === before, rich_message
  end

  after = exp.call

  rich_message = -> do
    code_string = expression.respond_to?(:call) ? _callable_to_source_string(expression) : expression
    error = "`#{code_string}` didn't change"
    error = "#{error}. It was already #{to.inspect}" if before == to
    error = "#{message}.\n#{error}" if message
    error
  end
  refute_equal before, after, rich_message

  unless to == UNTRACKED
    rich_message = -> do
      error = "Expected change to #{to.inspect}, got #{after.inspect}\n"
      error = "#{message}.\n#{error}" if message
      error
    end
    assert to === after, rich_message
  end

  retval
end