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Could anyone explain the difference between filter and filter_by functions in SQLAlchemy? Which one should I be using?
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The second one, filter_by(), may be used only for filtering by something specifically stated - a string or some number value. So it's usable only for category filtering, not for expression filtering. On the other hand filter() allows using comparison expressions (==, <, >, etc.)
first() applies a limit of one within the generated SQL, so that only one primary entity row is generated on the server side (note this may consist of multiple result rows if join-loaded collections are present). Calling Query. first() results in an execution of the underlying query.
The grouping is done with the group_by() query method, which takes the column to use for the grouping as an argument, same as the GROUP BY counterpart in SQL. The statement ends by calling subquery() , which tells SQLAlchemy that our intention for this query is to use it inside a bigger query instead of on its own.
all() will return all records which match our query as a list of objects.
filter_by is used for simple queries on the column names using regular kwargs, like
db.users.filter_by(name='Joe')
The same can be accomplished with filter, not using kwargs, but instead using the '==' equality operator, which has been overloaded on the db.users.name object:
db.users.filter(db.users.name=='Joe')
You can also write more powerful queries using filter, such as expressions like:
db.users.filter(or_(db.users.name=='Ryan', db.users.country=='England'))
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