django error on migration: "There is no unique constraint matching given keys for referenced table

Question

So I have seen that a lot of these kinds of questions have popped up (few answered) and none in a Django aspect that I saw. I am confused why I am getting the error, I am guessing i am missing something on my field decorator or what not in my model definition. Here are the two models... (one abbreviated). I thought I did everything right with unique and primary key set to true in the one table that the foreign key gives reference to but upon migrate I get this error:

django.db.utils.ProgrammingError: there is no unique constraint matching given keys for referenced table "swsite_zoneentity"

Edit up dated the code ...

class ZoneEntity(models.Model):
    zone_number = models.CharField(max_length=100, primary_key=True)
    mpoly = models.PolygonField() #this should grow and shrink for the most representative one...
    objects = models.GeoManager() 
    created_at=models.DateField(auto_now_add=True)
    updated_at=models.DateField(auto_now=True)

class CesiumEntity(models.Model):
    be_number = models.CharField(max_length=100) #the number assigned to a foot print to distinguish
    #zone_id = models.CharField(max_length=100, null=True, blank=True)
    zone_id = models.ForeignKey('ZoneEntity', null=True, blank=True)

Answer 1

To solve this, needed to add the unique constraint on the postgres table id myself.

psql <your-database-name>
ALTER TABLE swsite_zoneentity ADD CONSTRAINT zone_unique_id UNIQUE(id);

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Answer 2

This problem appears most times because you copied or created your database from a dump and somewhere the unique constraint on your primary key column(as well as other constraints got lost.

Solution:

Open your DB with pg4admin or any client,    Databases>your_database>schema>public>tables>your_table right-click   
on the table name, 
Choose Properties 
Select columns tabs 
switch primary key on your pk column
save/exit
run migration again