mongodb聚合查詢無法創建

Question

Hello all i have different document containing "category" field i have to get result from aggregation query to find number of document in each category and find result in array which sould contain {_id and num } field; 


    { 
     "_id": NumberInt(23), 
     "title": "WiredTiger T-shirt", 
     "slogan": "Unleash the tiger", 
     "description": "Crafted from ultra-soft combed cotton, this essential t-shirt features sporty contrast tipping and MongoDB's signature leaf.", 
     "stars": NumberInt(0), 
     "category": "Apparel", 
     "img_url": "/img/products/wt-shirt.jpg", 
     "price": 22 
    } 


    { 
     "_id": NumberInt(22), 
     "title": "Water Bottle", 
     "slogan": "Glass water bottle", 
     "description": "High quality glass bottle provides a healthier way to drink. Silicone sleeve provides a good grip, a see-through window, and protects the glass vesse [...]", 
     "stars": NumberInt(0), 
     "category": "Kitchen", 
     "img_url": "/img/products/water-bottle.jpg", 
     "price": 23 
    } 

我有很多這樣的文檔。我使用了下面的查詢： -

db.collectionname.find（{category：「categoryname」}，{_ id：1}）。count（）; 但在這個階段我只會得到一個類別的數據。

Answer 1

您必須爲此查詢使用組。對於小組，你必須在MongoDB中使用aggreagetion。下面將做你想要什麼：

db.collection.aggregate([{$group: {_id: "$category", num: {$sum: 1}}}]);

MongoDB的文檔也有一個例子： https://docs.mongodb.org/manual/reference/command/count/