bingo! thanks Simon. that's exactly the question :)well, the checks on the cube are already there (they must have a value higher then 0 to have a volume), but i must not increment the cube children more then it's maximum capacity. i'm considering an event listener as well, but i'm wondering if it can't be done at database level, using postgres?
or, what would be the better approach scenario for this? a trigger, an event, both?
On 05/19/2015 01:13 PM, Simon King wrote:
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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