Python - SqlAlchemy: Filter query by great circle distance?

Question

I am using Python and Sqlalchemy to store latitude and longitude values in a Sqlite database. I have created a hybrid method for my Location object,

@hybrid_method
def great_circle_distance(self, other):
    """
    Tries to calculate the great circle distance between the two locations

    If it succeeds, it will return the great-circle distance
    multiplied by 3959, which calculates the distance in miles.

    If it cannot, it will return None.

    """
    return math.acos(  self.cos_rad_lat 
                     * other.cos_rad_lat 
                     * math.cos(self.rad_lng - other.rad_lng)
                     + self.sin_rad_lat
                     * other.sin_rad_lat
                     ) * 3959

All the values like cos_rad_lat and sin_rad_lat are values I pre-calculated to optimize the calculation. Anyhow, when I run the following query,

pq = Session.query(model.Location).filter(model.Location.great_circle_distance(loc) < 10)

I get the following error,

line 809, in great_circle_distance
    * math.cos(self.rad_lng - other.rad_lng)
TypeError: a float is required

When I print the values for self.rad_lng and other.rad_lng I get, for example,

self.rad_lng: Location.rad_lng 
other.rad_lng: -1.29154947064

What am I doing wrong?

Answer 1

You can't really use the math module that way:

>>> c = toyschema.Contact()
>>> c.lat = 10
>>> c.lat
10
>>> import math
>>> math.cos(c.lat)
-0.83907152907645244
>>> math.cos(toyschema.Contact.lat)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: a float is required

You'll have combine sqalchemy.func.* in place of math.* in a @great_circle_distance.expression method for all of that kind of cleverness. Unfortunately, you can't do that with sqlite, either; it doesn't provide trig functions You could use PostgreSQL, which does, or you can try to add these functions to sqlite yourself:

EDIT It's actually not to hard to add functions to sqlite: This is NOT tested.

Have to add the math functions to sqlite:

engine = sqlalchemy.create_engine("sqlite:///:memory:/")
raw_con = engine.raw_connection()
raw_con.create_function("cos", 1, math.cos)
raw_con.create_function("acos", 1, math.acos)

class Location(...):
    ...
    @hybrid_method
    def great_circle_distance(self, other):
        """
        Tries to calculate the great circle distance between 
        the two locations by using the Haversine formula.

        If it succeeds, it will return the Haversine formula
        multiplied by 3959, which calculates the distance in miles.

        If it cannot, it will return None.

        """
        return math.acos(  self.cos_rad_lat 
                         * other.cos_rad_lat 
                         * math.cos(self.rad_lng - other.rad_lng)
                         + self.sin_rad_lat
                         * other.sin_rad_lat
                         ) * 3959

    @great_circle_distance.expression
    def great_circle_distance(cls, other):
        return sqlalchemy.func.acos(  cls.cos_rad_lat 
                         * other.cos_rad_lat 
                         * sqlalchemy.func.cos(cls.rad_lng - other.rad_lng)
                         + cls.sin_rad_lat
                         * other.sin_rad_lat
                         ) * 3959

Answer 2

Obviously, you cannot get a float from that string.

It is because you are using "self", which, as first parameter of the call, indicates that the method is a part of the object, and not some var you may pass on.

You should try this :

def great_circle_distance(self, first, other):
    """
    Tries to calculate the great circle distance between 
    the two locations by using the Haversine formula.

    If it succeeds, it will return the Haversine formula
    multiplied by 3959, which calculates the distance in miles.

    If it cannot, it will return None.

    """
    return math.acos(  self.cos_rad_lat 
                     * other.cos_rad_lat 
                     * math.cos(first.rad_lng - other.rad_lng)
                     + self.sin_rad_lat
                     * other.sin_rad_lat
                     ) * 3959

I suppose here above that the global variables "self.cos_rad_lat" and "self.sin_rad_lat" are initiated with correct values somewhere else in your program, probably in the "init" section of the same object.

Answer 3

It does look like you've done everything correctly, but somehow the method is not actually getting 'hybridized'. Could you have done something stupid, like not actually put the decorator on in your source code?