The AT LEAST ONE clause in FEKL is used to create a Fact Type with a Mandatory Role Constraint and an Internal Uniqueness Constraint on the far Role of the Fact Type.

E.g. The following FEKL statement with a ONE clause creates the Object-Role Model below:

FEKL: 


Object-Role Model:

NB If a Many-to-Many Relationship is required, rather than a 1-to-Many Relationship, the Internal Uniqueness Constraint on the Fact Type can be changed.
E.g. If you need the relationship to be reciprocal and where a Management Role can also have many Managers.

NB At this stage the Manager Entity Type has no Reference Mode and so is in error and coloured red, and similarly the Management Role Value Type has no Data Type, and so is in error and is red.