In the previous lesson, we saw how we could use match clauses in Insert queries in order to have new data reference existing data. In this lesson, we’ll see how we can use that strategy to perform polymorphic inserts. We have already created a new promotion entity for the Spring Sale with this query from a previous exercise.

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However, we have not yet assigned any books to this sale. This sale is a pretty big one, so we’re going to assign every book in the store to it with a 10% discount.

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By matching books using the supertype book, this query will add all books to the promotion, regardless of whether they are paperbacks, hardbacks, or ebooks. In this case, TypeDB is using type inference to resolve the inheritance polymorphism in the write query. We can just as easily use interface or parametric polymorphism in data writes!

In addition to inferring types of referenced data, TypeDB is able to infer the roles of roleplayers when inserting new relations. This allows us to write significantly more concise Insert queries. For example, in the following query, we insert rating and action-execution relations without specifying roles!

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For this query, TypeDB is able to infer that $review plays the review role and $frankenstein plays the rated role in rating. This is because:

These constraints mean that TypeDB can infer the roles of $review and $frankenstein unambiguously, as they could not play any other roles. The same applies to the action-execution relation. Conversely, if one or more of the roleplayers can play multiple roles, then TypeDB is unable to infer them and throws an exception. Consider the following query, for instance.

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The types country and state can play both location:location and location:located, so any of the following four combinations of roles would be valid:

It is obvious to us that the first statement is the correct one, but TypeDB cannot determine this unambiguously, so we would have to specify the roles when inserting this particular locating relation.