fix : fail to alter column from smallint to boolean

[jeffry-luqman]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

Fix USING ?::? sql for boolean field on AlterColumn func

Without this, it will report error as I described in the issue #164

User Case Description

If we want to change column from smallint to boolean, it will report error like below:

2023/02/28 14:03:35 /home/user/go/pkg/mod/gorm.io/driver/postgres@v1.4.8/migrator.go:318 ERROR: cannot cast type smallint to boolean (SQLSTATE 42846)
[0.374ms] [rows:0] ALTER TABLE "column_structs" ALTER COLUMN "is_active" TYPE boolean USING "is_active"::boolean

We only need to fix sql become USING xxx::int::boolean for boolean field in func AlterColumn in postgres@v1.4.8/migrator.go

[a631807682]

In fact, it should throw an error if the default rules fail, instead of us defining the conversion rules.

for example

type ColumnStruct struct {
name string
IsActive string // set IsActive to "true" and alter column using `::INT::boolean`
}

[jeffry-luqman]

In fact, it should throw an error if the default rules fail, instead of us defining the conversion rules.

for example

type ColumnStruct struct {
name string
IsActive string // set IsActive to "true" and alter column using `::INT::boolean`
}

OK, I just sent the fix with this sql, please review again

ALTER TABLE "column_structs" ALTER COLUMN "is_active" TYPE boolean USING CASE WHEN "is_active" IN ('1','TRUE','true','T','t') THEN true ELSE false END

[a631807682]

I may not have made it clear, we can automatically convert from string to boolean by ::boolean instead of ::INT:boolean.
We may need a function, possibly called genUsingExpression, to handle expressions that cannot be converted by default from type a to type b.
When type string to type boolean, use default conversion ::boolean, when type smallint to type boolean, use the specified conversion ::INT:boolean. If there are other types that have the same problem, we just need to fill in this method.

[jeffry-luqman]

ok, I've fixed it

[jeffry-luqman]

if there's nothing else to be fixed, can it be merged @a631807682 @jinzhu , please?

[jeffry-luqman]

close #164 at v1.5.0