没有过滤器的 Java MongoDB 3.0 驱动程序查询不同
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Java MongoDB 3.0 driver query distinct without filter
提问by ThisClark
How may I query distinctwith the Java MongoDB 3.0 driver?
如何使用 Java MongoDB 3.0 驱动程序查询不同？
I am attempting to query unique categoriesrecords from a locationscollection in MongoDB. In the Mongo shell, this is very simple: db.locations.distinct("categories");
我正在尝试从MongoDB 中的位置集合查询唯一类别记录。在 Mongo shell 中，这很简单：db.locations.distinct("categories");
In Java, it's not the same.
在 Java 中，情况不一样。
MongoClient client = new MongoClient();
MongoDatabase db = client.getDatabase("yelp");
//this will not compile, although examples from before 3.0 do it this way
MongoCursor<Document> c =
db.getCollection("locations").distinct("categories").iterator();
回答by jyemin
To let you avoid casts for distinct, the MongoCollection API lets you provide the expected type of the distinct values for the field. So if you know they are all strings, for example, you can write:
为了让您避免对 distinct 进行强制转换，MongoCollection API 允许您为字段提供不同值的预期类型。因此，例如，如果您知道它们都是字符串，则可以编写：
MongoCursor<String> c =
db.getCollection("locations").distinct("categories", String.class).iterator();
or all numbers:
或所有数字：
MongoCursor<Number> c =
db.getCollection("locations").distinct("categories", Number.class).iterator();
You can still do:
你仍然可以这样做：
MongoCursor<Object> c =
db.getCollection("locations").distinct("categories", Object.class).iterator();
if you can't guarantee anything about the types of the values for the field you're querying.
如果您不能保证您查询的字段的值的类型。
回答by ThisClark
I'll create an example database in the MongoDB shell and go as far as delivering a distinct query in Java with the 3.0 driver.
我将在 MongoDB shell 中创建一个示例数据库，并使用 3.0 驱动程序在 Java 中提供不同的查询。
In the shell, create a database with demo data:
在 shell 中，创建一个包含演示数据的数据库：
use imagedb;
db.createCollection("image_files");
db.image_files.insert({ file: "1.jpg", aspect: "4:3" });
db.image_files.insert({ file: "1.jpg", aspect: "16:9" });
db.image_files.insert({ file: "2.jpg", aspect: "4:3" });
db.image_files.insert({ file: "2.jpg", aspect: "16:9" });
db.image_files.insert({ file: "3.jpg", aspect: "2.35:1" });
In the shell, we may find distinct records.
在 shell 中，我们可能会发现不同的记录。
> db.image_files.distinct('file');
[ "1.jpg", "2.jpg", "3.jpg" ]
How to do it with Java and the MongoDB 3.0 driver?
如何使用 Java 和 MongoDB 3.0 驱动程序做到这一点？
import org.bson.Document;
import com.mongodb.MongoClient;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoCursor;
import com.mongodb.client.MongoDatabase;
public class Main {
public static void main(String[] args) {
MongoClient mongo = new MongoClient();
MongoDatabase db = mongo.getDatabase("imagedb");
MongoCollection<Document> filesCollection = db.getCollection("image_files");
MongoCursor<String> files = filesCollection.distinct("file", String.class).iterator();
while(files.hasNext()) {
System.out.println(files.next());
}
}
}
