Reference Composition

[soulstompp]

Currently the value of a composition is static which greatly limits query re-usability. The first attempts at increasing this re-usability were the :count() and :union() style commands but they were fairly limited and greatly increase code complexity (it generates SQL which requires knowing which dialect of SQL a driver wants.

After a fair amount of using these compositions I have found it would be much easier (as well as more flexible) to write a query with a subquery that could easily be swapped out. For example writing a generic count query that works similar to the :count() macro as follows:
SELECT COUNT(base.id) AS base_count FROM (:reference(ref_name)) base (referred to below as count_base.cql).

This query would compose whatever query ref_name is set to and count whatever column was specified as id.

This base.cql template could be used as follows:
SELECT base_count FROM :compose(count_base.cql SETTING ref_name TO :compose(active_users.cql)

Assuming simple.cql was SELECT id, name FROM users WHERE active = 1 this would result in:
SELECT COUNT(base.id) FROM (SELECT id, name FROM users WHERE active = 1) base allowing for a count query that is reusable for any other query with an id column and without having to alter the active_users.cql query in order to count these users.

If a query has a reference that is not provided by the calling macro using SETTING composition should fail.

If the SETTING keyword is used while composing a query without any references composition should also fail.

[spazm]

noob questions: :)

1. when you say 'value of a composition is static', do you mean that the results are fixed at parsing, or do you mean it has a 'static lifetime? Or the fact that the composer produces the actual SQL for the backend rather than having some sort of re-usable type that can be converted to SQL in a jit manner at the end.
2. Is one of the goals to rework :count and :union internally to use the :setting keyword?

anyways, now that I'm looking at the code again, going to see if I remember and finally understand ComposerTrait, and most of the crate types, SqlComposition, SqlCompositionAlias, ParsedItem etc.