python中的简单SQLAlchemy查询过滤器

Question

I'm trying to select all rows in a sql table where the first four characters of a text column match a certain string. (The backend database is a sqlite instance with limited column types, so bear with me)

The code I've written for the select is this:

    rows = SECtable.query.filter(str(SECtable.date)[:4] == str(matchingString)).all()

What am I doing wrong here? The query never matches any rows

Answer 1

If you use SECtable.date == 'some_string' , this produces an expression ( sqlalchemy.sql.expression.BinaryExpression ), which will be evaluated when you execute the query.

str(SECtable.date)[:4] == str(matchingString) is evaluated immediately, it produces the string representation of SECtable.date (i'd guess 'SECTable.date' ), and compares all but the fist for characters to str(matchingString) . so what you're writing here is basically:

'able.date' == str(matchingString)

which will probably evaluate to false, so you end up with filter(False) .

sqlalchemy provides a endswith functionality you could use in this case:

rows = SECtable.query.filter(SECtable.date.endswith(matchingString)).all()

Answer 2

You probably want to use SQLAlchemy's implementation of SQL's LIKE.

See the following documentation:

• http://docs.sqlalchemy.org/en/rel_0_8/orm/tutorial.html#common-filter-operators
• http://docs.sqlalchemy.org/en/rel_0_8/core/sqlelement.html#sqlalchemy.sql.operators.ColumnOperators.like
• http://docs.sqlalchemy.org/ru/latest/orm/query.html