From: Brian Candler <B.Candler@...>
Date: 2007-03-21T06:05:20+09:00
Subject: ActiveRecord, Oracle oci8 and LEFT OUTER JOIN

Hello,

This is an ActiveRecord question / issue. I hope it's OK to raise it here(*)

I'm using activerecord-1.15.2 as part of a Rails app. I started development
using Sqlite3 as the backend for expediency (laptop programming on a
plane!). I'm now trying to move it to Oracle 10.2. Unfortunately, the eager
loading of related objects using :include seems to generate SQL which Oracle
barfs on.

The error message I get is:
--------------------------------------------------------------------------
Showing app/views/entities/show.rhtml where line #1 raised:

OCIError: ORA-00920: invalid relational operator: SELECT entities.id AS
t0_r0, entities.key_attribute_id AS t0_r1, entities.key_value AS t0_r2,
entities.state AS t0_r3, avpairs.id AS t1_r0, avpairs.entity_id AS t1_r1,
avpairs.attribute_id AS t1_r2, avpairs.value AS t1_r3 FROM entities  LEFT
OUTER JOIN avpairs ON avpairs.entity_id = entities.id WHERE (0 OR
(entities.key_attribute_id=2 AND entities.key_value='vpn1s4') OR
(entities.key_attribute_id=3 AND entities.key_value='000001')) 
--------------------------------------------------------------------------

And the source which generates this query:

    cond = ["0"]
    links.each do |pri, key_attribute_id, key_value|
      cond.first << " OR (entities.key_attribute_id=? AND entities.key_value=?)"
      cond << key_attribute_id << key_value
    end
    entities = Entity.find(:all, :conditions => cond, :include => :avpairs)

The 'links' array includes a list of keys to Entity objects that I wish to
load, together with their linked Avpair objects. This all works just dandy
under Sqlite3.

Does this mean that AR eager loading can't work at all with Oracle? Or is
there something specific about this particular query which is causing the
problem?

In the "Agile" book I see a footnote on page 362:
"In fact, it might not work at all! If your database doesn’t support
left outer joins, you can’t use the feature. Oracle 8 users, for instance,
will need to upgrade to version 9 to use preloading."

But then I'd have thought I'd be fine with Oracle 10.2

The Oracle SQL reference starts at
http://download-uk.oracle.com/docs/cd/B19306_01/server.102/b14200/toc.htm
and the documentation for SELECT is at
http://download-uk.oracle.com/docs/cd/B19306_01/server.102/b14200/statements_10002.htm#i2065646

It shows:

  join_clause ::= table-reference (inner | outer_join_clause)+

  outer_join_clause ::= ... outer_join_type JOIN table_reference ON condition

  outer_join_type ::= (FULL|LEFT|RIGHT) OUTER

so I can't actually see why it would reject the SQL generated by AR. Any
ideas?

Regards,

Brian.

(*) At http://ar.rubyonrails.com/ it does say:

"For other information, feel free to ask on the ruby-talk mailing list
(which is mirrored to comp.lang.ruby) or contact david@loudthinking.com"

However if there's a more appropriate ActiveRecord mailing list please point
me at it. There are a couple of other minor AR issues/suggestions I'd like
to raise too.