sqlalchemy column that is sql expression with bind parameters - sqlalchemy

Question

I'm trying to map a class that has a column, that doesn't really exist, but is simply a sql expression that takes bind parameters at query time. The model below is an example of what I'm trying to do.
class House(db.Model):
__tablename__ = 'houses'
id = db.Column(db.Integer, primary_key=True)
#hybrid_property
def distance_from_current_location(self):
# what to return here?
return self.distance_from_current_location
#distance_from_current_location.expression
def distance_from_current_location(cls, latitude, longitude):
return db.func.earth_distance(
db.func.ll_to_earth(latitude, longitude), cls.earth_location)
# defer loading, as the raw value is pretty useless in python
earth_location = deferred(db.Column(EARTH))
Then I'd like to query via flask-sqlalchemy:
latidude = 80.20393
longitude = -90.44380
paged_data = \
House.query \
.bindparams({'latitude': latitude, 'longitude': longitude}) \
.paginate(1, 20, False)
My questions are:
How do I do this? Is it possible to use a hybrid_property like this?
If I can use a hybrid_property, what should the python method return? (there is no python way to interpret this, it should just return whatever the DB expression returned.
latitude and longitude only exist during query time and need to be bound for each query. How do I bind latitude and longitude during query time? The bindparams bit in my code snippet I just made up, but it illustrates what I want to do. Is it possible to do this?
I've read the docs but couldn't find any hybrid_property or method with a bind parameter in the examples...
(Also since this is not a real column, but just something I want to use on my model I don't want this to trigger alembic to generate a new column for it).
Thanks!

Answer 1

You can't do this. distance_from_current_location is not a fake column either, since it depends on query-specific parameters. Imagine you were to write a SQL view for this; how would you write the definition? (Hint: you can't.)
SQLAlchemy uses the identity map pattern, which means that for a particular primary key, only one instance exists in your entire session. How would you handle querying the same instance but with different latitude/longitude values? (The instances returned from the later query would overwrite those returned from the earlier one.)
The correct way to do this is through additional entities at query time, like this:
House.query \
.with_entities(House, db.func.earth_distance(db.func.ll_to_earth(latitude, longitude), House.earth_location)) \
.filter(...)
Or through a hyrbid_method (the usage of which requires passing in latitude and longitude every time):
class House(db.Model):
...
#hybrid_method
def distance_from_current_location(self, latitude, longitude):
# implement db.func.ll_to_earth(latitude, longitude), self.earth_location) **in Python** here
#distance_from_current_location.expression
def distance_from_current_location(cls, latitude, longitude):
...