java - 确保嵌套重复实体的索引

Question

我需要对嵌套文档实现唯一约束，例如:

urlEntities: [ { "url" : "http://t.co/ujBNNRWb0y" , "display_url" : "bit.ly/11JyiVp" ,  "expanded_url" :"http://bit.ly/11JyiVp"} , { "url" : "http://t.co/DeL6RiP8KR" , "display_url" : "ow.ly/i/2HC9x" , "expanded_url" : "http://ow.ly/i/2HC9x"}]

url, display_url, and expaned_url need to be unique. How to issue ensureIndex command for this condition in MongoDB?

Also, is it a good design to have nested documents like this or should I move them to a separate collection and refer them from here inside urlEntities? I'm new to MongoDB, any best practices suggestion would be much helpful.

Full Scenario:

Say if I have a document as below in the db which has millions of data:

{ "_id" : { "$oid" : "51f72afa3893686e0c406e19"} , "user" : "test" , "urlEntities" : [ { "url" : "http://t.co/64HBcYmn9g" , "display_url" : "ow.ly/nqlkP" , "expanded_url" : "http://ow.ly/nqlkP"}] , "count" : 0}

When I get another document with similar urlEntities object, I need to update user and count fields only. First I thought of enforcing unique constraint on urlEntities fields and then handle exception and then go for an update, else if I check for each entry whether it exists before inserting, it will have significant impact on the performance. So, how can I enforce uniqueness in urlEntities? I tried

{"urlEntities.display_url":1,"urlEntities.expanded_url":1},{unique:true}

但我仍然可以毫无异常(exception)地插入同一个文档两次。

Answer 1

仅对每个文档强制执行唯一性。您无法阻止以下情况(从您的示例中简化):

db.collection.ensureIndex( { 'urlEntities.url' : 1 } );
db.col.insert( {
    _id: 42,
    urlEntities: [
        { 
            "url" : "http://t.co/ujBNNRWb0y"
        },
        { 
            "url" : "http://t.co/ujBNNRWb0y"
        } 
    ]
});

同样，嵌套文档的复合唯一键也会遇到同样的问题。

您可以执行以下操作:

db.collection.insert( {
    _id: 43,
    title: "This is an example",
} );
db.collection.update( 
    { _id: 43 },
    {
        '$addToSet': { 
            urlEntities: { 
                "url" : "http://t.co/ujBNNRWb0y" , 
                "display_url" : "bit.ly/11JyiVp" ,  
                "expanded_url" : "http://bit.ly/11JyiVp"
            }
        }
    }
);

现在您已拥有 _id 43 的文档以及一个 urlEntities 文档。如果您再次运行相同的更新查询，它将不会添加新的数组元素，因为 url、display_url 和 Expanded_url 的完整组合已经存在存在。

此外，请查看 $addToSet 查询运算符的示例:http://docs.mongodb.org/manual/reference/operator/addToSet/