Add support for Criteria composition

[FlatMapIO]

Criteria.where(Condition) will support the construction of dynamic where clauses.

val noCondition = Conditions.just("1 = 1")
val condition = noCondition.and("name = :name").or(...)
client.select()
            .from("table")
            .matching(Criteria.where(condition))
            .fetch()

[mp911de]

Thanks for the ticket. These are actually two different requirements here. First, Criteria.where(…). The second one deals with the fact of introducing the SQL DSL API into the criteria API.

Spring Data R2DBC uses the SQL DSL on a lower abstraction level to create statements by considering dialect-specifics. The example name = :name does not work here. Named parameters are resolved only for plain queries via the execute(…) API. Furthermore,

client.select()
            .from("table")
            .matching(Criteria.where(condition))
            .fetch()

does not accept parameter bindings as we expect parameters to be passed in through the Criteria object. So there's no way how we could bind the parameter.

The Criteria API was built to help with simple use-cases. It makes sense to introduce an empty Criteria object via Criteria.empty() that allows composition without the code to consider whether you're constructing the first or a subsequent Criteria segment.