Affects Version/s: 2.0-ALPHA4
Fix Version/s: None
Security Level: All
This request is in followup to my doctrine-user message "Doctrine 2.0: Nested joins'.
I am a bit surprised by the responses in that defining precedences in joins by placing parenthesis around join expressions is not well-known. Although not in the original SQL92 specification it is a major and important feature offered by all the RDBMS's that Doctrine 2 supports, and oftenly performs better than using subselects or alike. Doctrine 1 did not support it, but imho Doctrine 2 should support it to be a mature allround ORM.
As a short example the following is a SQL statement with a nested join, where the nesting is absolutely necessary to return only a's together with either both b's and c's or no b's and c's at all:
FROM a A
LEFT JOIN (
INNER JOIN c C ON C.b_id = B.id
) ON B.a_id = A.id
In order for Doctrine 2 to support this the BNF should be something like:
Join ::= ["LEFT" ["OUTER"] | "INNER"] "JOIN" ( "(" JoinAssociationPathExpression ["AS"] AliasIdentificationVariable Join ")" | JoinAssociationPathExpression ["AS"] AliasIdentificationVariable ) [("ON" | "WITH") ConditionalExpression]
instead of the current:
Join ::= ["LEFT" ["OUTER"] | "INNER"] "JOIN" JoinAssociationPathExpression ["AS"] AliasIdentificationVariable [("ON" | "WITH") ConditionalExpression]
This would allow DQL like:
SELECT A, B, C
FROM a A
LEFT JOIN (
INNER JOIN B.c C
) WITH B.something = 'value' AND C.something = 'othervalue'
What further needs to be done is that the DQL parser loosly couples the ConditionalExpression to any of the previously parsed JoinAssociationPathExpression's instead of tieing it explicitely to the JoinAssociationPathExpression that preceedes it according to the old BNF notation. The new BNF should however not require any changes to the hydrator. Therefore I have the feeling that improving the DQL parser for nested joins does not require extensive work, while the benefit of running these kind of queries is considerable.
As an extra substantiation here are links to (BNF) FROM clause documentations of the RDBMS's that Doctrine 2 supports, they all show support for nested joins:
PostgreSQL: http://www.postgresql.org/docs/8.4/interactive/sql-select.html#SQL-FROM and http://www.postgresql.org/docs/8.1/interactive/explicit-joins.html
I surely hope you will consider implementing this improvement because it would save me and others from the hassle of writing raw SQL queries or executing multiple (thus slow) queries in DQL for doing the same. Thanks anyway for the great product so far!