What is the problem with "uninitialized constant"?

Question

Ruby on Rails

Good afternoon.
I updated the system, downloaded the project from Git and try to run it in the development environment.
When running migrations for the development environment, it gives an error

NameError: uninitialized constant

to one of the old migrations, the run is terminated.
The project is written in Ruby 2.2.2 Rails 4.2.0
The project is working and has been in production for a long time.
Tell me what could be the problem?

Answer 1

Look closely at the name of the models. Well, or show the code at least. The error is probably somewhere more specific than in your question.

Answer 2

Most likely, a model is called in the migration that was later deleted. On production, everything happened sequentially, first they ran the migrations, so they deleted the model. When passing migrations from scratch, a problem got out.