The active record pattern is an approach to accessing data in a database. A database table or view is wrapped into a class. Thus, an object instance is tied to a single row in the table. After creation of an object, a new row is added to the table upon save. Any object loaded gets its information from the database. When an object is updated, the corresponding row in the table is also updated. The wrapper class implements accessor methods or properties for each column in the table or view.
It was fashionable for a while to say it was an anti-pattern because that was a contrary view and ActiveRecord is very tied into building Rails applications.
It's not about fashion. Observations about fashion are no deeper than fashion itself.
It scales badly with table size, I think by design. That's why SQLAlchemy's and Hibernate's Data Mapper pattern is slightly more cumbersome to write, but works out much better.
It's a pattern where a single object (the "active record") not only represents a single database row, but also usually is responsible for saving/inserting data into the database (via save method) and retrieving them (via find methods). Because of this it breaks SRP. If this is neglectable or not, I don't want to argue here. Personally, I would not use it anymore because of bad experience in the past.
Active Record is NOT anti pattern. It's just one of ways to get things done related to (relational) database and your application. Active Record, Data Mapper, Raw SQL or whetever has its pros and cons.
