I had this problem because I changed the name of the class in a model, and it did not match the name of the file.
"Model class names use CamelCase. These are singular, and will map automatically to the plural database table name.
Model files go in app/models/#{singular_model_name}.rb."
https://gist.github.com/iangreenleaf/b206d09c587e8fc6399e#model