Mongoengine 简明教程

MongoEngine - QuerySet Methods

QuerySet 对象拥有以下用于查询数据库的有用方法。

The QuerySet object possesses following useful methods for querying the database.

first()

将返回满足查询的第一个文档。以下代码将返回 products 集合中的第一个文档，价格 < 20000。

First document satisfying the query is returned. Following code will return first document in products collection, that has price < 20000.

qset=products.objects(price__lt=20000)
doc=qset.first()
print ('Name:',doc.Name, 'Price:',doc.price)

Output

Name: Router Price: 2000

exclude()

这将导致从查询集中排除提到的字段。此处，Document 类的 to_json() 方法用于获取 Document 的 JSON 化版本。ProductID 字段不会出现在结果中。

This will cause mentioned fields to be excluded from Query Set. Here, to_json() mehod of Document class is used to obtain JSONified version of Document. ProductID field will not appear in the result.

for product in products.objects.exclude('ProductID'):
   print (product.to_json())

Output

{"_id": {"$oid": "5c8dec275405c12e3402423c"}, "Name": "Laptop", "price": 25000}
{"_id": {"$oid": "5c8dec275405c12e3402423d"}, "Name": "TV", "price": 50000}
{"_id": {"$oid": "5c8dec275405c12e3402423e"}, "Name": "Router", "price": 2000}
{"_id": {"$oid": "5c8dec275405c12e3402423f"}, "Name": "Scanner", "price": 5000}
{"_id": {"$oid": "5c8dec275405c12e34024240"}, "Name": "Printer", "price": 12500}

fields()

使用此方法控制加载到查询集中的字段。使用字段名作为关键字参数，并设置为 1 以包含，设置为 0 以排除。

Use this method to manipulate which fields to load in the query set. Use field names as keyword arguments and set to 1 to include, 0 to exclude.

for product in products.objects.fields(ProductID=1,price=1):
print (product.to_json())

Output

{"_id": {"$oid": "5c8dec275405c12e3402423c"}, "ProductID": 1, "price": 25000}
{"_id": {"$oid": "5c8dec275405c12e3402423d"}, "ProductID": 2, "price": 50000}
{"_id": {"$oid": "5c8dec275405c12e3402423e"}, "ProductID": 3, "price": 2000}
{"_id": {"$oid": "5c8dec275405c12e3402423f"}, "ProductID": 4, "price": 5000}
{"_id": {"$oid": "5c8dec275405c12e34024240"}, "ProductID": 5, "price": 12500}

在 fields() 方法中将字段关键字参数设置为 0 的效果类似于 exclude() 方法。

Setting field keyword argument to 0 in fields() method works similar to exclude() method.

for product in products.objects.fields(price=0):
print (product.to_json())

Output

{"_id": {"$oid": "5c8dec275405c12e3402423c"}, "ProductID": 1, "Name": "Laptop"}
{"_id": {"$oid": "5c8dec275405c12e3402423d"}, "ProductID": 2, "Name": "TV"}
{"_id": {"$oid": "5c8dec275405c12e3402423e"}, "ProductID": 3, "Name": "Router"}
{"_id": {"$oid": "5c8dec275405c12e3402423f"}, "ProductID": 4, "Name": "Scanner"}
{"_id": {"$oid": "5c8dec275405c12e34024240"}, "ProductID": 5, "Name": "Printer"}

only()

这种方法的效果类似于 fields() 方法。只有与关键字参数对应的字段才会出现在查询集中。

Effect of this method is similar to fields() method. Fields corresponding to keyword arguments only will appear in the query set.

for product in products.objects.only('Name'):
print (product.to_json())

Output

{"_id": {"$oid": "5c8dec275405c12e3402423c"}, "Name": "Laptop"}
{"_id": {"$oid": "5c8dec275405c12e3402423d"}, "Name": "TV"}
{"_id": {"$oid": "5c8dec275405c12e3402423e"}, "Name": "Router"}
{"_id": {"$oid": "5c8dec275405c12e3402423f"}, "Name": "Scanner"}
{"_id": {"$oid": "5c8dec275405c12e34024240"}, "Name": "Printer"}

sum()

此方法计算查询集中给定字段的总和。

This method computes sum of given field in the query set.

average()

此方法计算查询集中给定字段的平均值。

This method calculates average of given field in the query set.

avg=products.objects.average('price')
ttl=products.objects.sum('price')
print ('sum of price field',ttl)
print ('average of price field',avg)

Output

sum of price field 94500
average of price field 18900.0