Some background about my application:
Problem: if I run a command migrations:diff to generate migration from changes in my entity classes, I get the following exception:
[Doctrine\DBAL\DBALException] Unknown column type "json" requested.
Any Doctrine type that you use has to be registered with \Doctrine\DBAL\Types\Type::addType(). You can get a list of all the known types with \Doctrine\DBAL\Types\Type::getTypesMap().
The problem was because old column in database had a comment (DC2Type:json). I changed it to (DC2Type:json_array) and it works.
Why this problem occur? While doctrine is calculating the DIFF between entity mappings with database columns and actual database columns, Doctrine parses MySQL column comment ((DC2Type:json)) and validates json type. Since from newer Doctrine versions, you must use json_array instead of json, Doctrine library throws an exception.
The issue is that doctrine/dbal version 2.5.x had no support for JSON fields in MySQL.
Bump the version of doctrine/dbal in your composer.json to ^2.6 and it should work. Make sure to check for compatibility-breaking changes in the release changelog