On Tue, May 19, 2015 at 4:06 PM, Mike Bayer <mike...@zzzcomputing.com> wrote:
>
>
> On 5/19/15 10:54 AM, Richard Gerd Kuesters wrote:
>
> thanks Mike!
>
> when i stated about the limit, it was because it must not be taken as a
> parameter for any query, which "select * from blah" and "select * from bla
> limit N" should be return the same exactly number of rows, including where
> filters and so on. it is something like a "physical" rule, where my parent
> is (really) a box and the children its items (so, i cannot put more items
> then the box's limit).
>
> either way, creating a relationship with limit *can* provide me that sort of
> behaviour?
>
> I don't understand what "the behavior" is here. No LIMIT, yet there is a
> "limit". An assertion? I have no idea what you mean. The
> relationship 1. emits SQL 2. loads the results into objects. Are we
> talking about 1. or 2. ?
>
>
I'm guessing he's looking for something like this:
class Cube(Base):
__table_name__ = 'cube'
id = sa.Column(sa.Integer, primary_key=True)
rows = sa.Column(sa.Integer)
cols = sa.Column(sa.Integer)
depth = sa.Column(sa.Integer)
@property
def volume(self):
return self.rows * self.cols * self.depth
class Cell(Base):
__table_name__ = 'cell'
id = sa.Column(sa.Integer, primary_key=True)
cube_id = sa.Column(sa.ForeignKey(Cube.id))
row = sa.Column(sa.Integer)
col = sa.Column(sa.Integer)
depth = sa.Column(sa.Integer)
__table_args__ = (
sa.UniqueConstraint('cube_id', 'row', 'col', 'depth')
)
cube = saorm.relationship(Cube, backref='cells')
Now, given an instance of Cube, how can you ensure that it is
impossible to add more than Cube.volume cells to the Cube.cells
relationship? I imagine it is possible by attaching an event listener
to Cube.cells and doing the validation in there.
If Richard's data really is structured like this, I'd probably instead
want to enforce that:
0 <= cell.row < cell.cube.rows
0 <= cell.col < cell.cube.cols
0 <= cell.depth < cell.cube.depth
I think you could probably do this with SQLAlchemy validators. I don't
know enough Postgres, but I suspect you could also enforce it at the
database level.
Simon
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