I'm Jerry Wohlgemuth.

ActiveRecord often goes out of its way to guess your intent when querying. For example, it will convert strings to integers if that is the column type being queries.

You can easily check the output of the conversion like this:

Model.arel_table[:id].type_cast_for_database('1')
=> 1

In Rails 6, the serialization method changed. Maybe you were accepting some params from a controller and doing something like:

Model.where.not(id: ['temporary-id', 1, 2])

In Rails 5, the following query would be generated:

SELECT * FROM users WHERE id NOT IN (0, 1, 2)

This query would run as you expect and return all the records except 1 and 2 since the primary key id is ≥ 1.

In Rails 6, this query would be executed:

SELECT * FROM users WHERE id NOT IN (NULL, 1, 2)

Now, the result set is completely different. Because the string was serialized as NULL and the id field will never be NULL this will always return an empty relation.

This is kind of a fringe case and you probably shouldn't be passing in those string params in the first place, but serialization is something to watch out for when accepting user input and upgrading your Rails versions.