The bottom line is that I use migrations correctly, he does not, and
it creates problems in our workflow. I have mentioned it to him as
sort of a suggestion, but the response I received was “well, to each
his own.” False.
Agreed. But one clarification based on my years as a developer & now a Unix/Linux systems administrator. The vast majority of the work any of us does in the world of tech is invisible. We sit in front of a computer, tap some keys, something happens and that’s about it. There is an inherent isolation to this work that—key point—can mire some developers into idiosyncratic habits.
So this co-worker’s habits might be bad, but there might be a historic reason for their doing the work the way they are doing it.
Which does not mean it’s right. Because what you do on your own when coding is different from being part of a group or being a part of an organization.
They are a member of a team. And their behavior will negatively impact the work of others.
All I want is for my boss to ask my coworker to change this annoying
habit. I don't want to be a “tattle-tale” but I also don’t want to
have to continue cleaning up other people's slop.
First & foremost, your boss will never be able to change your co-worker’s habits. Seriously. They can reprimand your co-worker or perhaps facilitate a discussion between the two of you, but from the sounds of it your co-worker is simply an uncooperative member of the team at best.
How can you solve this? Well, make a strong case about the issue. Document each time you have had to clean up their work. Is this in a code repository? Even better. Then approach your boss not about asking for them to be dictating behavior, but rather talk to your boss & say: “Look, this is an issue. Let’s meet with him to discuss this.”
Then simply have all of you three meet to discuss why & how these conflicts in code are leading to workflow issues.
Also, be ready to not have anything solved. That’s life.
But you need to document their habits, make an effort to resolve them co-operatively and then if they screw up? The blame falls on them. Which would reflect on them & their future with the company.