Helper combinators for entity readers on entities

[dwhjames]

Suppose that there is a customer entity with a primaryAddress attribute that is a cardinality one reference to a address entity, and furthermore, that an address entity has a city attribute (cardinality one, string).

Currently in Datomisca, one would express an entity reader on customers that extracts the city of the primary address as:

primaryAddress.read[Entity].map(e => e.read[String](city))

With the andThen combinator, this can more succinctly be expressed as:

primaryAddress.read[Entity] andThen city.read[String]

Now suppose that a customer entity also has a cardinality many reference attribute addresses that references all address entities associated with the customer.

To get the set of all cities, one would currently write

addresses.read[Set[Entity]].map(_.map(e => e.read[String](city)))

With the andThenAll combinator, this can more succinctly be expressed as:

addresses.read[Set[Entity]] andThenAll city.read[String]

Finally, suppose that address entities have a telephones attribute (cardinality many, string).

Then to get the set of all telephone numbers, one would currently write

addresses.read[Set[Entity]].map(_.flatMap(e => e.read[Set[String]](telephones)))

With the andThenFlatten combinator, this can more succinctly be expressed as:

addresses.read[Set[Entity]] andThenFlatten telephones.read[Set[String]]

[rjsvaljean]

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