Defining

A boolean in OCL can be both Non-nullable and Nullable.

NonNullableBoolean : Boolean

NullableBoolean : Boolean?

As an attribute on a class that is saved to the database, almost always use non-nullable. Why? Because otherwise it will default to null and it's easy to mistakenly handle False and Null incorrectly.

Evaluating

Remember to evaluate booleans like this:

class.BooleanAttribute

not class.BooleanAttribute

not like this:

class.BooleanAttribute = True

class.BooleanAttribute = False

i.e. without an equal sign and True, False.

Mistakes We Have Seen Happen

class.BooleanAttribute->notEmpty

The expression will always return True because it will be converted to a list with one Boolean in it and that list will never be empty.