How do I convert an existing column to Foreign Key?

[Tom Tanner]

I have two models `model_a` and `model_b`, where their tables look something like this...

Table `a` looks something like this -- first row is column names, both contain characters:

    id |  tractce   | someString

    2  |  "0011900" | "Label here"

Table `b`:

    id | tractFIPS
    1  | "0011900"

Here's code from `models.py`:

    class model_a(models.Model):
    tractce = models.CharField(max_length=6)
    someString = models.TextField()


    class model_b(models.Model):
    tractFIPS = models.CharField(max_length=6)

I want to convert `model_b` so that the `tractFIPS` field is a foreign key referencing `model_a`. Something like this I guess...

    class model_b(models.Model):
    tractFIPS_FK = models.ForeignKey(model_a)

And the table for `model_b` would look something like this in the end:

    id |  tractFIPS_FK_id
    1  |  2

How do I convert `model_b`'s `tractFIPS` field to a foreign key with the correct values in them?