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With this change, migrations will raise an exception when defining an already defined column.

ActiveSupport::TimeZone#at now accepts an optional second argument, containing (fractional) microseconds, just like Ruby's Time::at does.

When requesting columns names from database adapters, ActiveRcord:Result used to dup/freeze column names. This change prefers using fstrings, which cuts down on repeat allocations.

Before this fix a duplicated child relations validation error didn't propagate to the parent record, but now it is all fixed.