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R1153
Project Action projects one or many Model Attribute
Model Attribute is projected by zero, one or many Project Action
Let's consider what would happen if a Project Action did not select any Model Attributes.
In the first case let's assume that the input is a Table Flow. The Type of the output Table Flow would be the same Table. This is equivalent to a single Table Flow, so the Project Action would not specify any real activity.
If, on the other hand, the input is an Instance Flow, the output would be a Table Flow with a complete set of corresponding Table Attributes. Here the output is typed differently and we could argue the value of a no-attribute Project Action as specifying the conversion of an Instance Flow into a corresponding Table Flow. Certainly that's not the intention of the relational project operation and what we really need is a more explicit way, i.e. Flow Conversion to convert Table and Instance Flows into one another if that's what we really want to do.
So we require that at least one Model Attribute be projected.
A given Model Attribute may or may not be involved in any number of Project Actions.
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