Fix retrieving default value for text column for MariaDB

[fatkodima]

Fixes #38218
Fixes #41240

All adapters support text columns with defaults, except MySQL (but MariaDB >= 10.2.1 supports).

[intrip]

@fatkodima @kamipo I didn't see your PR and accidentally created a similar one #41726 🤦‍♂️

I'll close my PR buy if you want you can port the extra test that I've added for the SchemaDumper:

rails/activerecord/test/cases/schema_dumper_test.rb

Lines 622 to 626 in f6453e2

	def test_schema_dump_with_text_column
	output = dump_table_schema("dump_defaults")
	assert_match %r{t\.text\s+"text_with_default",.*?default: "John' Doe"}, output
	end if supports_default_text?

[janko]

Would love to see this one merged 🤘. Currently this issue is preventing us from setting default values for serialized JSON columns on MariaDB.

[lfalcao]

👀

[intrip]

@fatkodima since you're the author could you please fix the conflicts? Also please check #40822 (comment) incase you'd like to pull in the extra tests.

If you can't work on this I don't mind opening a new PR.

[fatkodima]

Rebased and pulled the test case.
Wow, got a nice build number on Buildkite - https://buildkite.com/rails/rails/builds/77777 😄

[zzak]

Is gsub the right operation here? I wonder if this might be costly 🤔

[fatkodima]

Did you mean replace it with gsub!? text columns with default are not popular enough, I think, and default values in the db most of the time have small values in them.

[zzak]

@fatkodima Sorry I was misconnecting the dots here. This code probably isn't being called during a request so maybe we don't have to worry about the performance of gsub. Please correct me if I'm wrong! 🙇

[fatkodima]

It is used in

rails/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

Lines 114 to 119 in 8a7a9d0

	def columns(table_name)
	table_name = table_name.to_s
	column_definitions(table_name).map do |field|
	new_column_from_field(table_name, field)
	end
	end

which runs once for each table, when columns list for the first time is needed, and then cached in SchemaCache -

rails/activerecord/lib/active_record/connection_adapters/schema_cache.rb

Lines 110 to 114 in 8a7a9d0

	def columns(table_name)
	@columns.fetch(table_name) do
	@columns[deep_deduplicate(table_name)] = deep_deduplicate(connection.columns(table_name))
	end
	end

So, no need to worry about the performance.

[kmooventhan98]

Why Mysql2 not supporting this?

Mysql2::Error: BLOB, TEXT, GEOMETRY or JSON column 'my_field_name' can't have a default value

[byroot]

This error is returned by the MySQL server. You won't get an answer on this thread.