assert_no_difference(expression, message = nil, &block) public

Assertion that the numeric result of evaluating an expression is not changed before and after invoking the passed in block. 

  assert_no_difference 'Article.count' do
    post :create, :article => invalid_attributes
  end

A error message can be specified. 

  assert_no_difference 'Article.count', "An Article should not be destroyed" do
    post :create, :article => invalid_attributes
  end
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July 2, 2010
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Takes array

Like assert_difference this method can take an array of expressions to evaluate all of them. For example:

assert_no_difference ['Publisher.count', 'User.count', 'Membership.count'] do
  post :create
end

It creates an assertion for each item in the array. So this will add three assertions to your test.