Meteor：在集合中仅返回嵌套数组中的单个对象(Meteor: Return only single object in nested array within collection)

 我试图用Meteor的find().fetch()过滤返回的数据集，只包含一个对象，如果我查询单个子文档但是我收到几个，有些甚至不包含任何子文档，它看起来不是很有用匹配条款。 
 
 我有一个简单的混合数据集合，如下所示： 
 
{
    "_id" : ObjectId("570d20de3ae6b49a54ee01e7"),
    "name" : "Entertainment",
    "items" : [
        {
            "_id" : ObjectId("57a38b5f2bd9ac8225caff06"),
            "slug" : "this-is-a-long-slug",
            "title" : "This is a title"
        },
        {
            "_id" : ObjectId("57a38b835ac9e2efc0fa09c6"),
            "slug" : "mc",
            "title" : "Technology"
        }
    ]
}
{
    "_id" : ObjectId("570d20de3ae6b49a54ee01e8"),
    "name" : "Sitewide",
    "items" : [
        {
            "_id" : ObjectId("57a38bc75ac9e2efc0fa09c9"),
            "slug" : "example",
            "name" : "Single Example"
        }
    ]
}
 
 我可以使用MongoDB shell轻松查询嵌套items数组中的特定对象，如下所示： 
 
db.categories.find( { "items.slug": "mc" }, { "items.$": 1 } );
 
 这会返回好的数据，它只包含我想要使用的单个对象： 
 
{
    "_id" : ObjectId("570d20de3ae6b49a54ee01e7"),
    "items" : [
        {
            "_id" : ObjectId("57a38b985ac9e2efc0fa09c8")
            "slug" : "mc",
            "name" : "Single Example"
        }
     ]
}
 
 但是，如果直接尝试Meteor中的类似查询： 
 
/* server/publications.js */
Meteor.publish('categories.all', function () {
    return Categories.find({}, { sort: { position: 1 } });
});
/* imports/ui/page.js */
Template.page.onCreated(function () {
    this.subscribe('categories.all');
});
Template.page.helpers({
    items: function () {
        var item = Categories.find(
            { "items.slug": "mc" },
            { "items.$": 1 } )
        .fetch();
        console.log('item: %o', item);
     }
 });
 
 结果并不理想，因为它返回整个匹配的块，以及嵌套items数组中的每个对象： 
 
{
    "_id" : ObjectId("570d20de3ae6b49a54ee01e7"),
    "name" : "Entertainment",
    "boards" : [
        {
            "_id" : ObjectId("57a38b5f2bd9ac8225caff06")
            "slug" : "this-is-a-long-slug",
            "name" : "This is a title"
        },
        {
            "_id" : ObjectId("57a38b835ac9e2efc0fa09c6")
            "slug" : "mc",
            "name" : "Technology"
        }
    ]
}
 
 然后我当然可以使用for循环进一步过滤返回的游标以获得所需的对象，但是在处理更大的数据集时，这似乎是不可扩展的并且非常低效。 
 
 我无法理解为什么Meteor的find返回一个完全不同于MongoDB shell find的数据集，唯一合理的解释是两个函数签名都不同。 
 
 我应该将嵌套集合分解为更小的集合并采用更多关系数据库方法（即存储对ObjectID的引用）和查询集合到集合的数据，还是有更强大的方法可用于有效地将大数据集过滤成单个如上所示，只包含匹配对象的对象？ 

I'm attempting to filter returned data sets with Meteor's find().fetch() to contain just a single object, it doesn't appear very useful if I query for a single subdocument but instead I receive several, some not even containing any of the matched terms.
 
I have a simple mixed data collection that looks like this:
 
{
    "_id" : ObjectId("570d20de3ae6b49a54ee01e7"),
    "name" : "Entertainment",
    "items" : [
        {
            "_id" : ObjectId("57a38b5f2bd9ac8225caff06"),
            "slug" : "this-is-a-long-slug",
            "title" : "This is a title"
        },
        {
            "_id" : ObjectId("57a38b835ac9e2efc0fa09c6"),
            "slug" : "mc",
            "title" : "Technology"
        }
    ]
}
{
    "_id" : ObjectId("570d20de3ae6b49a54ee01e8"),
    "name" : "Sitewide",
    "items" : [
        {
            "_id" : ObjectId("57a38bc75ac9e2efc0fa09c9"),
            "slug" : "example",
            "name" : "Single Example"
        }
    ]
}
 
I can easily query for a specific object in the nested items array with the MongoDB shell as this:
 
db.categories.find( { "items.slug": "mc" }, { "items.$": 1 } );
 
This returns good data, it contains just the single object I want to work with:
 
{
    "_id" : ObjectId("570d20de3ae6b49a54ee01e7"),
    "items" : [
        {
            "_id" : ObjectId("57a38b985ac9e2efc0fa09c8")
            "slug" : "mc",
            "name" : "Single Example"
        }
     ]
}
 
However, if a similar query within Meteor is directly attempted:
 
/* server/publications.js */
Meteor.publish('categories.all', function () {
    return Categories.find({}, { sort: { position: 1 } });
});
/* imports/ui/page.js */
Template.page.onCreated(function () {
    this.subscribe('categories.all');
});
Template.page.helpers({
    items: function () {
        var item = Categories.find(
            { "items.slug": "mc" },
            { "items.$": 1 } )
        .fetch();
        console.log('item: %o', item);
     }
 });
 
The outcome isn't ideal as it returns the entire matched block, as well as every object in the nested items array:
 
{
    "_id" : ObjectId("570d20de3ae6b49a54ee01e7"),
    "name" : "Entertainment",
    "boards" : [
        {
            "_id" : ObjectId("57a38b5f2bd9ac8225caff06")
            "slug" : "this-is-a-long-slug",
            "name" : "This is a title"
        },
        {
            "_id" : ObjectId("57a38b835ac9e2efc0fa09c6")
            "slug" : "mc",
            "name" : "Technology"
        }
    ]
}
 
I can then of course filter the returned cursor even further with a for loop to get just the needed object, but this seems unscalable and terribly inefficient while dealing with larger data sets.
 
I can't grasp why Meteor's find returns a completely different set of data than MongoDB's shell find, the only reasonable explanation is both function signatures are different.
 
Should I break up my nested collections into smaller collections and take a more relational database approach (i.e. store references to ObjectIDs) and query data from collection-to-collection, or is there a more powerful means available to efficiently filter large data sets into single objects that contain just the matched objects as demonstrated above?

原文：https://stackoverflow.com/questions/38794278