Get a list of values of one column from the results of a query
Question:
I have a User model where users can have the same name. I want to get the email addresses of all the users with a given name. However, I have to do result[0].email on the result of the query to get just the email for a row. I could do this with a for loop, but is there a way to just get the list of one field without having to do this every time?
my_result = db.session.query(my_table).filter_by(name=name)
emails = []
for r in my_result:
emails.append(r.email)
Answers:
No, there’s not really a way around the fact that SQLAlchemy queries return a list of dicts. If you want a specific set of fields, you can query for just those fields, but if you want a list of one field, you’ll have to extract it from the result. The following example gets a list of the unique emails for every user with the given name.
emails = [r.email for r in db.session.query(my_table.c.email).filter_by(name=name).distinct()]
Just to keep a record, I like a wrapper with this function in my common use lib:
def flat_list(l):
return ["%s" % v for v in l]
Then:
flat_list(db.session.query(Model.column_attribute).all())
There is a way to return specific columns from a filter_by query using the values method. As follows:
emails = [r[0] for r in db.session.query(my_table).filter_by(name=name).values('email')]
or:
emails = [r[0] for r in User.query.filter_by(name=name).values('email')]
values() takes any number of field names as parameters and returns a generator that has tuples with each value from each field name. Using a list comprehension to take the first item of that tuple emails will then be a list of plain string email addresses.
Please note that the values method is deprecated and you should use with_only_columns in the future.
Example to get only column b:
s = select(table1.c.a, table1.c.b)
s = s.with_only_columns(table1.c.b)
From the SQLAlchemy documentation for Result.scalars():
>>> result = conn.execute(text("select int_id from table"))
>>> result.scalars().all()
[1, 2, 3]
I have a User model where users can have the same name. I want to get the email addresses of all the users with a given name. However, I have to do result[0].email on the result of the query to get just the email for a row. I could do this with a for loop, but is there a way to just get the list of one field without having to do this every time?
my_result = db.session.query(my_table).filter_by(name=name)
emails = []
for r in my_result:
emails.append(r.email)
No, there’s not really a way around the fact that SQLAlchemy queries return a list of dicts. If you want a specific set of fields, you can query for just those fields, but if you want a list of one field, you’ll have to extract it from the result. The following example gets a list of the unique emails for every user with the given name.
emails = [r.email for r in db.session.query(my_table.c.email).filter_by(name=name).distinct()]
Just to keep a record, I like a wrapper with this function in my common use lib:
def flat_list(l):
return ["%s" % v for v in l]
Then:
flat_list(db.session.query(Model.column_attribute).all())
There is a way to return specific columns from a filter_by query using the values method. As follows:
emails = [r[0] for r in db.session.query(my_table).filter_by(name=name).values('email')]
or:
emails = [r[0] for r in User.query.filter_by(name=name).values('email')]
values() takes any number of field names as parameters and returns a generator that has tuples with each value from each field name. Using a list comprehension to take the first item of that tuple emails will then be a list of plain string email addresses.
Please note that the values method is deprecated and you should use with_only_columns in the future.
Example to get only column b:
s = select(table1.c.a, table1.c.b)
s = s.with_only_columns(table1.c.b)
From the SQLAlchemy documentation for Result.scalars():
>>> result = conn.execute(text("select int_id from table"))
>>> result.scalars().all()
[1, 2, 3]