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solid_assert is a simple implementation of an assert utility in Ruby. It lets you write tests for your assumptions while coding.

Assertions are meant to test conditions about the integrity of your code. You should use them for testing assumptions like the following:

  • If the flow reaches here, then this variable has to have this value.
  • This line of code should never be executed.
  • At this point, this list should contain one entry for each key in this hash.


Add to your Gemfile:

gem "solid_assert"


You can enable/disable assertions with:


Assertions are disabled by default and are typically used in development mode only. You might want to disable them in production for performance reasons.

Use assert for testing conditions. You can optionally provide an error message.

assert some_string != "some value"
assert clients.empty?, "The list must not be empty!"

Use invariant for testing blocks of code. This comes handy when testing your assumptions requires several lines of code. You can provide an optional message too.

invariant do
  one_variable = calculate_some_value
  other_variable = calculate_some_other_value
  one_variable > other_variable
invariant "Lists must have equal sizes!" do
  len = calculate_list_length
  other_len = calculate_other_list_length
  len == other_len

Assertion Error

Failed assertion will raise SolidAssert::AssertionFailedError error. You shouldn't catch it in a rescue block! If it raised then something is wrong with either your code or with you assumption. Assertions shouldn't be used for handling error situations! Use Ruby built-in exception handling for that.


Create a file named solid_assert.rb in the config/initializers dir with the following content:

SolidAssert.enable_assertions unless Rails.env.production?

This way assertions will be disabled in production and enabled in the rest of environments.