How to change column type and all foreign keys in PostgreSQL?

Question

I'm currently stuck in a catch 22 with my Django Application. I have to change the type of a column from Varying char to Integer as i'm moving from UUID's to plain old ID's (the data is not changing ever as it's physical constants). Now django threw a Fit originally about not being able to cast from VarChar to Int, so i "helped" it out with:

 ALTER TABLE glass_fill ALTER COLUMN id TYPE INTEGER USING CAST(id AS INT);

Now, it says:

django.db.utils.DatabaseError: foreign key constraint "glass_fill_manufacturer_glass_fill_id_fkey" cannot be implemented
DETAIL:  Key columns "glass_fill_id" and "id" are of incompatible types: integer and character varying.

Any Ideas?

Note: The glass_fill_manufacturer Table hasn't been created yet django trys to on syncdb but fails. also the,

ALTER TABLE glass_fill ALTER COLUMN id TYPE INTEGER USING CAST(id AS INT); 

line didn't alter the column as I thought.

glass_fill table schema:

-- Table: glass_fill

-- DROP TABLE glass_fill;

CREATE TABLE glass_fill
(
  id character varying(36) NOT NULL,
  name character varying(255),
  temperature real,
  density real,
  viscosity real,
  conductivity real,
  heat_capacity real,
  colour text,
  CONSTRAINT glass_fill_pkey PRIMARY KEY (id )
)
WITH (
  OIDS=FALSE
);

Answer 1

ALTER TABLE glass_fill_manufacturer 
ALTER COLUMN glass_fill_id TYPE INTEGER USING CAST(glass_fill_id AS INT)
;

I guess by your text that Django will somehow do it but if it doesn't:

ALTER TABLE glass_fill_manufacturer 
drop constraint glass_fill_manufacturer_glass_fill_id_fkey
;

alter table glass_fill_manufacturer 
ADD constraint glass_fill_manufacturer_glass_fill_id_fkey 
foreign key (glass_fill_id) references glass_fill (id)
;

Answer 2

You should do the following:

1. Drop the constraints
2. Create a temporary table to hold mapping between old and new ids
3. Update glass_fill, returning the values into the temporary table
4. Update the referencing tables from the temporary table
5. Alter column types on all tables
6. Recreate the constraints