访问对象列表上的唯一闭包会产生UnsupportedOperationException

Question

我正在使用Grails 2.3.11并使用MongoDb插件mongodb:3.0.3.

我的代码看起来像这样:

def home() {
   List abcs = ABC.list()
   println abcs.unique { it.parent }
}

访问唯一闭包时代码中断.

例外说:

ERROR errors.GrailsExceptionResolver- UnsupportedOperationException   occurred when processing request: [GET] /MongoAppInEcomEnv/product/home
Stacktrace follows:
Message: null
Line | Method
->>  148 | add       in java.util.AbstractList
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
|    108 | add       in     ''
|    342 | addAll .  in java.util.AbstractCollection
|    120 | $tt__home in com.test.aitm.ProductController$$EPCH6LtA
|    198 | doFilter  in grails.plugin.cache.web.filter.PageFragmentCachingFilter
|     63 | doFilter  in grails.plugin.cache.web.filter.AbstractFilter
|   1145 | runWorker in java.util.concurrent.ThreadPoolExecutor
|    615 | run       in java.util.concurrent.ThreadPoolExecutor$Worker
^    744 | run . . . in java.lang.Thread

Answer 1

看起来列表是空的。

你也可以尝试

abcs.parent.unique()