Fix: Long column identifiers in deeply nested joins cause fields to be omitted

[IgorStepanov]

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

What did this pull request do?

Fixes #7513

User Case Description

PostgreSQL has 63-character limit for identifiers (including select field aliases).
In a long chain of joins we may have a troubles:

err = db.Model(&Table1{}).Debug().
	Joins("Looooooooooooooooooooooooooooooooooooooooooooooooooooooooong").
        Joins("Looooooooooooooooooooooooooooooooooooooooooooooooooooooooong.Table2").
        Joins("Looooooooooooooooooooooooooooooooooooooooooooooooooooooooong.Table2.Table3").Find(&objList).Error

This code truncates long aliases similar Namer.UniqueName and stores a truncated to original map in db.Statement that allow to not touch old fileld mapping strategy.

[maxwey]

Thank you for taking a crack at this bug!

I left a couple comments, but am not a maintainer (or even very familiar with Gorm internals)

[maxwey]

Postgres (can't speak for others) does not limit names to 63 characters, but instead limits it to 63 bytes. Counting runes instead of bytes can mean an incorrect truncation limit.

For example, using the query:

SELECT id AS "aręęęľlly_long_name_that_isnt_64_runes_long_is_still_truncated" FROM forms LIMIT 1;

Yields the results:

aręęęľlly_long_name_that_isnt_64_runes_long_is_still_trunca
1

Which you'll note is still being truncated, despite it being only 62 runes long.
Here's a go playground with the differences.
It appears within the playground linked above that the truncation that yields similar results to Postgres is the simple 63 byte truncation, not 63 rune truncation.

As a "fun" edge case, if a multi-byte utf8 character is the final character in the string and being truncated, Postgres appears to drop the invalid character (though doing a simple identifer[:63] would result in the last byte being invalid).

e.g.:

SELECT id AS "a_multi_byte_character_that_is_64_runes__long__is__truncated__漢" FROM forms LIMIT 1;

Results

a_multi_byte_character_that_is_64_runes__long__is__truncated__
1

Mutli byte ending - Go playground

(All my testing was done with Postgres 13.20)

[IgorStepanov]

Hmm. Thanks. I'll fix it today.

[IgorStepanov]

Fixed

[maxwey]

Would it be clearer to name this TruncatedAliases since not all aliases being added to the map are field aliases, as seen here?

[IgorStepanov]

Done