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由 hotatekaoru 提交于
To change a NOT NULL constraint `reversible`. When changing a NOT NULL constraint, we use `ActiveRecord::ConnectionAdapters::SchemaStatements#change` method that is not reversible, so `up` and `down` methods were required. Actually, we can use `change_column_null` method if only one constraint changed, but if we want to change multiple constarints with ALTER QUERY, `up` and `down` methods were required.
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