Multiple column key and "keyless" pivot models

Is your feature request related to a problem? Please describe.
Currently there is no way to have a model without an id field. For pivot models and in some other cases you want to have multiple identifier fields instead of a single id field.

Describe the solution you'd like
Adding id = true to multiple @Column annotations should result in all fields being used as the model identifier.

该 issue 将 @Column 注解和模型标识符处理确定为入口。首先定位处理 id=true 的 Java 代码，并检查现有的标识符行为；完成的标准是支持多个标识符字段和不带 id 的模型，同时保持当前行为不变。