自定义比较的一个具体示例显示在GeoAlchemy虽然它可能是矫枉过正的你的应用程序）。虽然以下链接中的示例使用边界框来比较空间坐标，但您可以使用更简单的方法在容差范围内进行比较。

请参阅比较器类在这里定义了〜=和其他操作符：

http://geoalchemy-2.readthedocs.org/en/0.2.4/spatial_operators.html

既然你提及您无法找到如何实现自定义比较功能的文档，请参阅此处：

http://docs.sqlalchemy.org/en/latest/core/types.html#types-operators

在相关说明中 - 如果要防止数据库更新在浮点列保持在某个阈值内时触发，请参阅此处的讨论：

https://groups.google.com/forum/#!msg/sqlalchemy/tCpsGZmjk_w/lOwW93qHV0sJ

（免责声明：这是基于谷歌搜索的，我是SQLAlchemy的新手）

To summarize the problem, I can not select any entity against a floating point value using SQLAlchemy.

For example:

m = session.query(Model).get(1);
all_m = session.query(Model).filter(Model.some_float_value, m.some_float_value)

all_m is empty while I would expect it to always have at least one!

How can I filter a float value in SQLAlchemy, based on an arbitrary precision (EG. some I may want to match to 0.01 or others I may filter with a precision of 0.0005).

For example, I want to be able to write a generic function so I can write queries like this in my code:

session.query(Model)\
    .filter(Model.foo == "bar",
            match_float(Model.some_float_value, float_val, 0.025)).all()

Where there tolerance of the matches anything within an approximate tolerance of 0.025.

However, I am not very familiar with SQLAlchemy, and was unable to locate documentation on how to create custom compare function, or any built in functions that would do what I need.

Is there a built in function I can use, a way to provide a custom compare function for the filter method, or do I a way to write a raw query/procedure for this?

Aditional Info:

Postgres 9.2

SQLAlchemy 0.9

Values are stored as double precision floating point

All Models were defined like so (there are a lot of them, which makes a solution with explicit column definitions less than desirable):

engine = create_engine('postgresql://user:pass@localhost:1234/database')
Base = declarative_base()
metadata = MetaData(bind=engine)

class Model(Base):
    __table__ = Table('model', metadata, autoload=True)

edit

Iv'e completely reworded my question for clarity. Sorry for the confusion, I fail at communicating with humans...

One concrete example for custom comparisons is shown in GeoAlchemy (though it may be overkill for your application). While the examples in the link below use bounding boxes for comparing spatial coordinates, you could use a simpler approach to implement comparison within a tolerance.

See for example the Comparator class which defines "~=" and other operators here:

http://geoalchemy-2.readthedocs.org/en/0.2.4/spatial_operators.html

Since you mention that you were unable to find documentation on how to implement a custom compare function, see here:

http://docs.sqlalchemy.org/en/latest/core/types.html#types-operators

On a related note - if you want to prevent database updates from being triggered if a floating point column remains within some threshold, see the discussion here:

https://groups.google.com/forum/#!msg/sqlalchemy/tCpsGZmjk_w/lOwW93qHV0sJ

(Disclaimer: This is based on a google search, I am new to SQLAlchemy)