How to Correctly Filter SQLAlchemy Date Fields for Age Range Queries?

SQLAlchemy: Filtering Date Fields

In SQLAlchemy, filtering based on date fields requires careful consideration. Let's explore how to correctly filter between two dates to extract data within a specified age range.

Modeling the Date Field

Assuming you have a User model with a birthday field defined as a Date datatype, it is represented in the database as a string in the format 'YYYY-MM-DD', e.g., '1987-01-17'.

Filtering by Age Range

To select all users within a specific age range, let's say 18 to 30 years, we need to compare their birthdays to the corresponding dates that represent the start and end of that range.

Incorrect Approach

Your initial query, although conceptually correct, contains a typo that reverses the inequality symbols:

query = DBSession.query(User).filter(
  and_(User.birthday >= '1988-01-17', User.birthday <= '1985-01-17')
)

This query excludes all records because it checks for users whose birthdays are both greater than or equal to '1988-01-17' and less than or equal to '1985-01-17', which is impossible.

Correct Approach

To correctly filter, simply reverse the inequality symbols:

qry = DBSession.query(User).filter(
  and_(User.birthday <= '1988-01-17', User.birthday >= '1985-01-17')
)

Alternatively, you can use the built-in between() method:

qry = DBSession.query(User).filter(User.birthday.between('1985-01-17', '1988-01-17'))

Both approaches will return users whose birthdays fall within the specified date range, representing the desired age interval.

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