Rails - gmaps4rails gem on postgres

问题:

I successfully use the gmaps4rails gem on my local MySQL machine. However, when I deploy to PG on Heroku, I get the following error with respect to code that uses the gmaps4rails "near" function to find locations near the selected location:

2012-05-21T17:58:40+00:00 app[web.1]: ActiveRecord::StatementInvalid (PG::Error: ERROR:  operator does not exist: numeric - character varying
2012-05-21T17:58:40+00:00 app[web.1]:                                                              ^
2012-05-21T17:58:40+00:00 app[web.1]: LINE 1: ...8.755864232 * 2 * ASIN(SQRT(POWER(SIN((30.1926300 - venues.l...
2012-05-21T17:58:40+00:00 app[web.1]: HINT:  No operator matches the given name and argument type(s). You might need to add explicit type casts.
2012-05-21T17:58:40+00:00 app[web.1]: : SELECT  venues.*, 3958.755864232 * 2 * ASIN(SQRT(POWER(SIN((30.1926300 - venues.latitude) * PI() / 180 / 2), 2) + COS(30.1926300 * PI() / 180) * COS(venues.latitude * PI() / 180) * POWER(SIN((-85.8356740 - venues.longitude) * PI() / 180 / 2), 2) )) AS distance, CAST(DEGREES(ATAN2( RADIANS(longitude - -85.8356740), RADIANS(latitude - 30.1926300))) + 360 AS decimal) % 360 AS bearing FROM "venues"  WHERE (3958.755864232 * 2 * ASIN(SQRT(POWER(SIN((30.1926300 - venues.latitude) * PI() / 180 / 2), 2) + COS(30.1926300 * PI() / 180) * COS(venues.latitude * PI() / 180) * POWER(SIN((-85.8356740 - venues.longitude) * PI() / 180 / 2), 2) )) <= 5) ORDER BY distance LIMIT 5):
2012-05-21T17:58:40+00:00 app[web.1]:   app/controllers/venues_controller.rb:22:in `show'

I suspect that this is because of something not being supported in this query in postgres, but the gem supposedly supports postgres. Any idea what's going on?

回答1:

Looks like PostgreSQL is complaining about this:

30.1926300 - venues.latitude

and the error message says that there is no operator that allows you to subtract a string from a number. I'd guess that you've created your venues.latitude column as a :string when it should be a :float or :decimal. MySQL tries to be friendly be doing a lot of implicit type conversions behind your back, PostgreSQL tries to be friendly by making you say exactly what you mean to avoid confusion.

You're going to have to change your latitude column to a numeric type. Then you should start developing on top of PostgreSQL if you're going to deploy on top of Heroku's PostgreSQL, you should also match the PostgreSQL version in your development and deployment environments.

AFAIK, you'll have to change the type manually with an ALTER TABLE as a simple change_column in a migration will probably fail with an error similar to

column "latitude" cannot be cast to type double precision

A migration like this:

def up
    connection.execute(%q{
        alter table venues
        alter column latitude
        type float using latitude::float
    })
end

should do the trick for PostgreSQL. Presumably you'll have to fix venues.longitude as well.