Re: [Sqlalchemy-tickets] [sqlalchemy] #2914: sqlalchemy.orm.mapper.Mapper.attrs doesn't maintain order

[sqlalchemy <mi...@zz...>]

#2914: sqlalchemy.orm.mapper.Mapper.attrs doesn't maintain order
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      Reporter:  tisdall           |      Owner:  zzzeek
          Type:  defect            |     Status:  closed
      Priority:  low               |  Milestone:  0.9.xx
     Component:  orm               |   Severity:  minor - half an hour
    Resolution:  worksforme        |   Keywords:
Progress State:  completed/closed  |
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Comment (by zzzeek):

 the order of columns when using Declarative is determined by the order in
 which each Column is constructed, since otherwise the ordering of items on
 a class in Python is unordered.  So there is actually significant logic in
 order to maintain this ordering within the Table object that's created.

 This ordering counter is also shared on relationship objects and such, but
 declarative does not pass the objects to the mapper in such a way that
 this ordering is maintained; nor does it specify column/relationship
 objects as individuals, instead the mapper maps the whole Table object at
 once, so there is no way right now that an "interleaved" pattern of plain
 columns/relationships can be produced using standard declarative, unless
 declarative re-implemented the mapper's usual job of iterating through
 table columns which is redundant.   the ordering logic thus far is only
 concerned with making sure column order on the Table is maintained, as
 this results in DDL passed to the database.

 probably the most expedient way to sort in object creation order is this:

 {{{
 from sqlalchemy import inspect
 from sqlalchemy.orm import ColumnProperty
 mapper = inspect(Person)

 def creation_order(obj):
     if isinstance(obj, ColumnProperty):
         return obj.columns[0]._creation_order
     else:
         return obj._creation_order

 mapper._props.sort(key=lambda k: creation_order(mapper._props[k]))
 }}}

 why exactly would it be important such that the order in which attributes
 are declared on a class be mirrored when inspecting the mapping?  This is
 not the case for Python builtins such as `dir()`.  The attributes on a
 Python class are not ordered.

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Ticket URL: <http://www.sqlalchemy.org/trac/ticket/2914#comment:6>
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