assert_difference

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, article: {...}
end

An arbitrary expression is passed in and evaluated.

assert_difference 'assigns(:article).comments(:reload).size' do
  post :create, comment: {...}
end

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

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

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

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

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

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

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

An error message can be specified.

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