Postgres DB Column Type Conversion - ruby-on-rails

Question

Trying to convert a string column to date. Column currently holds values, such as '20170130', and '20161130'.
When I try:
change_column :tickets, :string_date, :date
This gives me an error:
PG::DatatypeMismatch: ERROR: column "string_date" cannot be cast automatically to type date.
HINT: Specify a USING expression to perform the conversion.
So, I try:
change_column :tickets, :string_date, 'date USING CAST(string_date AS date)'
Still no luck, with the following error:
PG::InvalidDatetimeFormat: ERROR: invalid input syntax for type date
Anyone know what's going on?? Thought converting from string to date would be easier...
thanks in advance

Answer 1

strange - I would expect to see the value, where you fail to cast date, eg:
t=# create table a2(string_date text);
CREATE TABLE
t=# insert into a2 values ('20170130'),('20161130');
INSERT 0 2
t=# insert into a2 select 'bad value';
INSERT 0 1
t=# alter table a2 alter COLUMN string_date type date using string_date::date;
ERROR: invalid input syntax for type date: "bad value"
maybe its an empty string?..
anyway - find bad value and fix it, then try to convert again:
t=# select string_date from a2 where string_date !~ '\d{8}';
string_date
-------------
bad value
(1 row)
t=# begin;
BEGIN
t=# delete from a2 where string_date !~ '\d{8}';
DELETE 1
t=# alter table a2 alter COLUMN string_date type date using string_date::date;
ALTER TABLE
t=# end;
COMMIT