On 7/6/15 12:06 PM, Ryan Holmes wrote:
I have an interesting problem that I haven't been able to solve for
quite some time. I keep finding information about association proxies
and the like, but nothing that really helps (or maybe I'm implementing
it incorrectly).
Let's start with this:
|
classHandledProjectedItemList(list):
defappend(self,proj):
proj.projected =True
list.append(self,proj)
item_table =Table("items",saveddata_meta,
Column("ID",Integer,primary_key =True),
Column("itemID",Integer,nullable =False,index =True),
Column("name",String,nullable =False),
Column("timestamp",Integer,nullable =False),
)
projectedItem_table =Table("projectedItem",saveddata_meta,
Column("sourceID",ForeignKey("item.ID"),primary_key =True),
Column("destID",ForeignKey("item.ID"),primary_key =True),
Column("enabled",Integer))
mapper(Item,item_table,
properties ={"projectedItem":relation(Item,
primaryjoin =projectedItem_table.c.destID
==item_table.c.ID,
secondaryjoin =item_table.c.ID
==projectedItem_table.c.sourceID,
secondary =projectedItem_table,
collection_class =HandledProjectedItemList)
})
|
I have two tables: a `item` table, and a `projectedItem` table. The
`item` table is the main one, and contains information on items. The
projected items table is a self-referential many-to-many relationship
to the items table, where each item may have a collection of other
items attached to it. We use a custom collection class to load this
relationship so that we can modify a special attribute in the Item
objects (if they are loaded via this relationship, they have their
`projected` attribute set to `True`).
This works great. But I also want read / write access to that extra
`enabled` column in the relationship table, while maintaining the
current functionality of loading the projected items into this custom
collection class. I haven't found any information on that that helps,
or like I said before, maybe I'm just not using it correctly.
you would make a new class ProjectedItem and map it to the
projectedItem_table. This is the association object pattern (get that
to work first, without the "proxy" part), illustrated at
http://docs.sqlalchemy.org/en/rel_1_0/orm/basic_relationships.html#association-object.
Once that works, and all that's left is the inconvenience of navigating
from Item->projecteditems->item, then you can apply the association
proxy pattern to convert those two hops into just one
(http://docs.sqlalchemy.org/en/rel_1_0/orm/extensions/associationproxy.html).
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