查找数组字段不为空的MongoDB记录
本文翻译自：Find MongoDB records where array field is not empty
All of my records have a field called "pictures". 我所有的记录都有一个名为“图片”的字段。 This field is an array of strings. 该字段是字符串数组。
I now want the newest 10 records where this array IS NOT empty. 我现在想要该数组不为空的最新10条记录。
I've googled around, but strangely enough I haven't found much on this. 我在Google上四处搜寻，但奇怪的是，我在这方面没有发现太多。 I've read into the $where option, but I was wondering how slow that is to native functions, and if there is a better solution. 我已经读过$ where选项，但是我想知道这对本机函数有多慢，以及是否有更好的解决方案。
And even then, that does not work: 即使这样，它也不起作用：
ME.find({$where: 'this.pictures.length > 0'}).sort('-created').limit(10).execFind()
Returns nothing. 不返回任何内容。 Leaving this.pictures without the length bit does work, but then it also returns empty records, of course. 留下this.pictures没有长度位的确可以，但是当然它也返回空记录。
#1楼
参考：https://stackoom.com/question/103TI/查找数组字段不为空的MongoDB记录
#2楼
经过一番摸索之后，尤其是在mongodb文档中，又使人困惑不解，这就是答案：
ME.find({pictures: {$exists: true, $not: {$size: 0}}})
#3楼
这也可能对您有用：
ME.find({'pictures.0': {$exists: true}});
#4楼
If you also have documents that don't have the key, you can use: 如果您还有没有密钥的文档，则可以使用：
ME.find({ pictures: { $exists: true, $not: {$size: 0} } })
MongoDB don't use indexes if $size is involved, so here is a better solution: 如果涉及$ size，MongoDB不会使用索引，因此这是一个更好的解决方案：
ME.find({ pictures: { $exists: true, $ne: [] } })
Since MongoDB 2.6 release, you can compare with the operator $gt but could lead to unexpected results (you can find a detailled explanation in this answer ): 从MongoDB 2.6版本开始，您可以与运算符$gt进行比较，但可能会导致意外的结果（您可以在此答案中找到详细的说明）：
ME.find({ pictures: { $gt: [] } })
#5楼
Starting with the 2.6 release, another way to do this is to compare the field to an empty array: 从2.6版本开始，另一种方法是将字段与一个空数组进行比较：
ME.find({pictures: {$gt: []}})
Testing it out in the shell: 在外壳中进行测试：
> db.ME.insert([
{pictures: [1,2,3]},
{pictures: []},
{pictures: ['']},
{pictures: [0]},
{pictures: 1},
{foobar: 1}
])
> db.ME.find({pictures: {$gt: []}})
{ "_id": ObjectId("54d4d9ff96340090b6c1c4a7"), "pictures": [ 1, 2, 3 ] }
{ "_id": ObjectId("54d4d9ff96340090b6c1c4a9"), "pictures": [ "" ] }
{ "_id": ObjectId("54d4d9ff96340090b6c1c4aa"), "pictures": [ 0 ] }
So it properly includes the docs where pictures has at least one array element, and excludes the docs where pictures is either an empty array, not an array, or missing. 因此，它适当地包括pictures至少具有一个数组元素的docs，并排除pictures为空数组，不是数组或缺失的docs。
#6楼
ME.find({pictures: {$exists: true}})
就这么简单，这对我有用。