Hi,
I've encountered a couple of instances of a specific SQL design
pattern, aimed at mimicking a linked list (of comments and sub-
comments to blog entries) and I'd just like to check that the pattern
is known not to be supported in ActiveMapper.
Michael Bayer describes the pattern as "handling a hierarchical
relationship on itself", and in his implementation, "contains a lazy
reference both to its parent comment and its list of child comments."
1st instance is reflog's Turboblog, uses SQLObject and handles the
mapping explicitly:
class Comment(SQLObject):
class sqlmeta:
table = "turboblog_comment"
[...]
parent_id = IntCol(default=-1)
sub_comments = RelatedJoin("Comment",joinColumn="parent_id")
2nd instance is Michael Bayer's zblog, uses SQLAlchemy, mapping is
handled via foreign_key:
comments = Table('comments', metadata,
Column('comment_id', Integer, primary_key=True),
[...]
Column('parent_comment_id', Integer, ForeignKey
('comments.comment_id')),
)
augmented with:
# comment mapper. This mapper is handling a hierarchical
relationship on itself, and contains
# a lazy reference both to its parent comment and its list of child
comments.
mapper(Comment, tables.comments, properties={
'id':tables.comments.c.comment_id,
'post':relation(Post, lazy=True, backref=backref('comments',
cascade="all, delete-orphan")),
'user':relation(user.User, lazy=False, backref=backref
('comments', cascade="all, delete-orphan")),
'parent':relation(Comment,
primaryjoin=tables.comments.c.parent_comment_id==tables.comments.c.comme
nt_id, foreignkey=tables.comments.c.comment_id, lazy=True,
uselist=False),
I've tried both approaches in ActiveMapper, neither of which seem to
work (but I'm a newbie w.r.t sqlalchemy):
class Comment(ActiveMapper):
class mapping:
__table__= "comment"
comment_id = column(Integer, primary_key=True)
[...]
parent_id = column(Integer, default=-1)
sub_comments = one_to_many("Comment", colname="parent_id",
backref="comment_id")
ArgumentError: Cant find any foreign key relationships between
'comment' and 'comment'
class Comment(ActiveMapper):
class mapping:
__table__= "comment"
comment_id = column(Integer, primary_key=True)
[...]
parent_id = column(Integer,
foreign_key="comment.comment_id", index=True)
sub_comments = one_to_many("Comment", colname="parent_id",
backref="comment_id")
AttributeError: 'ColumnProperty' object has no attribute
'_get_target_class'
I'm thinking that this design pattern may be outside the intentions
of ActiveMapper --- Michael Bayer has already indicated that
"ActiveMapper is just giving you a thin layer of "shortcut" on top of
things".
(It's not that it can't be done with an explicit Table+mapper, it's
just that I'm lazy and prefer to work at the highest level of
abstraction available if possible.)
Cheers,
Graham Higgins.
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