65 lines
3.0 KiB
ReStructuredText
65 lines
3.0 KiB
ReStructuredText
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.. _performing_joins_in_select:
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Perfoming joins in select
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=========================
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.. versionadded:: 7.0
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Starting with OpenERP 7.0, an ``_auto_join`` attribute is added on *many2one* and
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*one2many* fields. The purpose is to allow the automatic generation of joins in
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select queries. This attribute is set to False by default, therefore not changing
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the default behavior of those fields. It is not recommended to use this attribute
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unless you understand the limitations of the feature.
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Without ``_auto_join``, the behavior of expression.parse() is the same as before.
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Leafs holding a path beginning with many2one or one2many fields perform a search
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on the relational table. The result is then used to replace the leaf content.
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For example, if you have on res.partner a domain like ``[('bank_ids.name',
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'like', 'foo')]`` with bank_ids linking to res.partner.bank, 3 queries will be
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performed :
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- 1 on res_partner_bank, with domain ``[('name', '=', 'foo')]``, that returns a
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list of (res.partner.bank) bids
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- 1 on res_partner, with a domain ``['bank_ids', 'in', bids)]``, that returns a
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list of (res.partner) pids
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- 1 on res_partner, with a domain ``[('id', 'in', pids)]``
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When the _auto_join attribute is True, it will perform a select on res_partner
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as well as on res_partner_bank.
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- the relational table will be accessed through an alias: ``'"res_partner_bank"
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as res_partner__bank_ids``
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- the relational table will have a join condition on the main table:
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``res_partner__bank_ids."partner_id"=res_partner."id"``
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- the condition will be written on the relational table:
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``res_partner__bank_ids."name" = 'foo'``
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This job is performed in expression.parse(). For leafs containing a path, it
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checks whether the first item of the path is a *many2one* or *one2many* field
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with the ``auto_join`` attribute set. If set, it adds a join query and recursively
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analyzes the remaining of the leaf, going back to the normal behavior when
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not reaching an ``_auto_join`` field. The sql condition created from the leaf
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will be updated to take into account the table aliases.
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Chaining _auto_join allows to reduce the number of queries performed, and to
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avoid having too long ``('id', 'in', ids)`` replacement leafs in domains.
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However, severe limitations exist on this feature that limits its current use as
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of version 7.0. **This feature is therefore considered as experimental, and used
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to speedup some precise bottlenecks in OpenERP**.
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List of known issues and limitations:
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- using _auto_join bypasses the business logic; no name search is performed, only
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direct matches between ids using join conditions
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- ir.rules are not taken into account when performing the _auto_join.
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- support of active_test is not asserted
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- support of translation is not asserted
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- support of _auto_join leading to function fields
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Typical use in OpenERP 7.0:
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- in mail module: notification_ids field on mail_message, allowing to speedup
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the display of the various mailboxes
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- in mail module: message_ids field on mail_thread, allowing to speedup the
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display of needaction counters and documents having unread messages
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