Rails & Postgres: Migration to change_colomn gives error “cannot be cast to type timestamp without time zone”

Question

A Rails migration to turn a \"deleted_at\" time column to a datetime column failed. Any ideas on how to solve this? It\'s a fresh install of Postgres if that is relevant.

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Answer 1

You can't alter a field's type from time to timestamp ("datetime"), because the values couldn't be converted -- the database doesn't know the date.

You can, however, drop and re-create the column:

ALTER TABLE products DROP COLUMN deleted_at;
ALTER TABLE products ADD COLUMN deleted_at timestamp;

Or if this field was set to NOT NULL, you should instead do:

ALTER TABLE products ADD COLUMN deleted_at timestamp NOT NULL;

But if you insist on retaining fake values in this table like Sean, you can use ALTER...TYPE...USING like this:

ALTER TABLE products ALTER COLUMN deleted_at TYPE timestamp USING
    CASE WHEN deleted_at IS NOT NULL THEN timestamp '1970-01-01 00:00:00' END;
-- Or:
ALTER TABLE products ALTER COLUMN deleted_at
    TYPE timestamp USING date '1970-01-01' + deleted_at;