What if my professor writes me a negative LOR, in order to keep me working with him? I am a undergrad student, and I am currently conducting summer research under a professor.

During our last meeting, I have revealed that I wish to go to school A for my graduate study. But my prof rather wanted me to stay at my current school for master/phd; i.e., stay and work for him.

Well, I have already spent 3 years in my current undergrad school and I know it too well it is not where I want to go.

The problem is that, this prof is the only person who can comment on my research experience/potential.  (Because I have been working 2 years with him; this year is the second year).  

Right now, I can sense something not right in the relationship between my prof and I. At the end, if I go ahead and ask him to write me a recommendation letter, I am sure he would write one. But the question is, there can be a chance that he writes negatively, such that I would get rejected everywhere, and have nowhere to go except staying at my current school and do graduate study with him. 

What should I do?



More detail info:

There used to be a PhD student who published quality paper and taking care of things in the lab. The Prof is almost always away so he definely need someone to do this kind of lab managing job. As far as I know, none of his graduate students publish paper for 3 years in a row except for that Phd student. Currently, that PhD student is graduated this year so I guess he was looking for a candidate as a replacement.
Writing bad letters of recommendation because of personal disappointment about a student leaving is so disproportionately unethical that I cannot believe that it happens with any significant frequency. It is not what I would be worried about.

In the end, most professors take pride in their students growing up, going out into the world, and being successful. Students are a bit our kids too -- sure, it would be nice if the kids stayed in the neighborhood, could continue to mow the lawn, and come over for dinner twice a week; but if they move out and are successful, we're proud nonetheless and hope they stay in contact. And if there's something we can help for them to be successful (like writing letters of recommendation), then that's what we do!