详解python中mongoengine库用法
目录
- 一、MongoDB的安装与连接
- 二、MongoEngine模型介绍
- 2.1、ODM模型介绍
- 2.2、常见数据类型
- 2.3、数据类型通用参数
- 2.4、类属性meta常见配置项
- 2.5、文档的嵌套模型
- 三、添加数据
- 3.1、方式一
- 3.2、方式二:使用create()方法
- 四、查询数据
- 4.1、单个文档查询
- 4.2、条件查询
- 4.3、聚合统计
- 4.4、排序
- 4.5、分页处理
- 五、修改和删除数据
- 5.1、修改数据
- 5.2、删除数据
一、MongoDB的安装与连接
安装:pip install mongoengine
连接mongodb数据库
from pymongo import MongoClient cli = MongoClient('192.168.124.104', 27017) mongodb = cli['test'] print(mongodb)
from mongoengine import connect # 连接到User数据库 connect('user', host='192.168.124.104', port=27017)
二、MongoEngine模型介绍
2.1、ODM模型介绍
from mongoengine import Document, StringField class User(Document): email = StringField(required=True) first_name = StringField(max_length=50) last_name = StringField(max_length=50)
2.2、常见数据类型
2.3、数据类型通用参数
- db_field:文档中的field/域/列名称
- required:是否为必填项
- default:默认值
- unique:唯一性约束
- choices:可选择的列表
- primary_key:是否为文档的主键,默认为False
2.4、类属性meta常见配置项
类属性,其配置项为python的dict(字典)
from mongoengine import Document, StringField class User(Document): email = StringField(required=True) first_name = StringField(max_length=50) last_name = StringField(max_length=50) meta = { }
常见的配置项:
- db_alias:指定文档所在的数据库(逻辑库)
- collection:指定文档所在的集合
- ordering:指定文档的默认排序规则
- indexes:指定文档的索引规则
2.5、文档的嵌套模型
学生信息数据字典
文档的嵌套场景
情况一,数组-简单数据类型:{'grades': [76, 51, 84]}
from mongoengine import Document, IntField, ListField class Student(Document): grade = ListField(IntField())
情况二,单个文档:{'grade': {'course_name': '语文', 'score': 76}}
from mongoengine import Document, StringField, IntField, ListField, EmbeddedDocument, EmbeddedDocumentField # 自定义类型 class CourseGrade(EmbeddedDocument): course_name = StringField() score = IntField() class Student(Document): grade = EmbeddedDocumentField(CourseGrade)
情况三,数组-文档:{'grades': [{'score': 76}, {'score': 51}]}
from mongoengine import Document, IntField, ListField, EmbeddedDocument, EmbeddedDocumentField # 自定义类型 class CourseGrade(EmbeddedDocument): score = IntField() class Student(Document): grade = ListField(EmbeddedDocumentField(CourseGrade))
完整代码:
from mongoengine import Document, connect, EnumField, StringField, IntField, ListField, EmbeddedDocument, \ EmbeddedDocumentField from enum import Enum # 连接到User数据库 connect('user', host='192.168.124.104', port=27017) class SexEnum(Enum): MAN = '男' WOMAN = '女' class CourseGrade(EmbeddedDocument): """成绩信息(科目、老师、成绩)-被嵌套的文档""" course_name = StringField(max_length=64, required=True, verbose_name='科目') teacher = StringField(max_length=16, verbose_name='老师') score = IntField(min_value=0, max_value=150, required=True, verbose_name='成绩') def __repr__(self): return f"CourseGrade({self.course_name},{self.score})" def __str__(self): return self.__repr__() class Student(Document): """学生信息""" # verbose_name 自定义参数,用于显示定义域的名称 stu_no = IntField(required=True, unique=True, verbose_name='学号') stu_name = StringField(required=True, max_length=16, verbose_name='姓名') sex = EnumField(enum=SexEnum, verbose_name='性别') class_name = StringField(max_length=10, verbose_name='班级') address = StringField(max_length=255, verbose_name='家庭住址') phone_no = StringField(max_length=11, verbose_name='电话号码') age = IntField(min_value=0, max_value=150, verbose_name='年龄') grades = ListField(EmbeddedDocumentField(CourseGrade), verbose_name='成绩数组') meta = { # 指定文档的集合 'collection': 'students', # 指定排序,可以指定多个域。例如:'age':根据年龄升序,'-age':根据年龄降序 'ordering': ['-age'] } def __repr__(self): return f'Grade({self.stu_no}, {self.stu_name})' def __str__(self): return self.__repr__() class Grade(Document): """学生成绩""" # verbose_name 自定义参数,用于显示定义域的名称 stu_no = IntField(required=True, verbose_name="学号") stu_name = StringField(required=True, max_length=16, verbose_name='姓名') sex = EnumField(enum=SexEnum, verbose_name='性别') class_name = StringField(max_length=10, verbose_name='班级') address = StringField(max_length=255, verbose_name='家庭住址') phone_no = StringField(max_length=11, verbose_name='电话号码') age = IntField(min_value=0, max_value=150, verbose_name='年龄') grade = EmbeddedDocumentField(CourseGrade, verbose_name='成绩') meta = { # 指定文档的集合 'collection': 'grades', # 指定排序,可以指定多个域。例如:'age':根据年龄升序,'-age':根据年龄降序 'ordering': ['-age'] } def __repr__(self): return f'Grade({self.stu_no}, {self.stu_name})' def __str__(self): return self.__repr__()
三、添加数据
添加数据一般有两种方式:
3.1、方式一
一般步骤:
- 第一步,构造ODM模型类对象:
user=User(username='张三')
- 第二步,验证数据:
user.validate()
- 第三步,保存数据:
user.save()
模型中的验
- 内置的验证器,如
max_length, min_value
- 自定义验证器
模型中自定义验证方法示例代码如下:
import re from mongoengine import StringField from mongoengine.errors import ValidationError def phone_required(value): pattern = r'^1[0-9][10]$' if not re.search(pattern, value): raise ValidationError('请输入正确的手机号') phone_no = StringField(validation=phone_required)
方式一示例代码:
import random from model import Student, Grade, SexEnum, CourseGrade class LearnMongoDBEngine(object): def __init__(self, info): self.info = info print(self.info) def add_one_student(self): """新增一个学生信息""" student = Student( stu_no=random.randint(3000, 9999999), stu_name=self.info['name'], sex=random.choice([SexEnum.MAN, SexEnum.WOMAN]), class_name='六年级三班', address=self.info['address'], phone_no=self.info['phone'], age=random.randint(10, 15), grades=[ CourseGrade(course_name='语文', teacher=self.info['teacher'], score=random.randint(1, 100)), CourseGrade(course_name='数学', teacher=self.info['teacher'], score=random.randint(1, 100)), CourseGrade(course_name='英语', teacher=self.info['teacher'], score=random.randint(1, 100)), ] ) print(student, student.grades) result = student.save() return result if __name__ == '__main__': info = {'name': '铁扇公主', 'address': '北京市朝阳区', 'phone': '19121741234', 'teacher': '王老师'} obj = LearnMongoDBEngine(info) obj.add_one_student()
mongodb数据库中插入的数据:
3.2、方式二:使用create()方法
User.objects.create(**kwargs)
示例代码:
import random from model import Student, Grade, SexEnum, CourseGrade class LearnMongoDBEngine(object): def __init__(self, info): self.info = info print(self.info) def add_one_student(self): """新增一个学生信息""" result = Student.objects.create( stu_no=random.randint(3000, 9999999), stu_name=self.info['name'], sex=random.choice([SexEnum.MAN, SexEnum.WOMAN]), class_name='六年级三班', address=self.info['address'], phone_no=self.info['phone'], age=random.randint(10, 15), grades=[ CourseGrade(course_name='语文', teacher=self.info['teacher'], score=random.randint(1, 100)), CourseGrade(course_name='数学', teacher=self.info['teacher'], score=random.randint(1, 100)), CourseGrade(course_name='英语', teacher=self.info['teacher'], score=random.randint(1, 100)), ] ) # student = Student( # stu_no=random.randint(3000, 9999999), # stu_name=self.info['name'], # sex=random.choice([SexEnum.MAN, SexEnum.WOMAN]), # class_name='六年级三班', # address=self.info['address'], # phone_no=self.info['phone'], # age=random.randint(10, 15), # grades=[ # CourseGrade(course_name='语文', teacher=self.info['teacher'], score=random.randint(1, 100)), # CourseGrade(course_name='数学', teacher=self.info['teacher'], score=random.randint(1, 100)), # CourseGrade(course_name='英语', teacher=self.info['teacher'], score=random.randint(1, 100)), # ] # ) print(result, result.grades) # result = student.save() return result if __name__ == '__main__': info = {'name': '卢俊义', 'address': '上海市浦东新区', 'phone': '18721741234', 'teacher': '张老师'} obj = LearnMongoDBEngine(info) obj.add_one_student()
mongodb数据库中插入的数据:
四、查询数据
结果集QuerySet的获取:User.objects
,User是模型对象
常用的查询方法:
- all():查询所有文档
- filter():按照条件查询
- count():满足条件的文档数
- sum()/average():求和/求平均数
- order_by():排序
- .skip().limit():分页
4.1、单个文档查询
- first():没有文档则返回None,User.objects.first()
- get(**kwargs)
多个文档时,异常:MultipleObjectsReturned
没有文档时,异常:DoesNotExist
仅有一个文档时:返回ODM对象
示例代码:
from model import Student class LearnMongoDBEngine(object): def get_one_student(self): """查询一个学生的信息""" student_info = Student.objects.first() print(student_info, student_info.id, student_info.stu_name, student_info.address) return student_info def get_all_student(self): """查询所有学生的信息""" student_all_info = Student.objects.all() print(student_all_info) return student_all_info def get_student_by_id(self, pk: str): """根据学生的id查询""" student_info_id = Student.objects.get(id=pk) print(student_info_id) return student_info_id if __name__ == '__main__': obj = LearnMongoDBEngine() obj.get_one_student() obj.get_all_student() obj.get_student_by_id('62dcd1f1a0da9e5521e73223')
运行结果:
4.2、条件查询
1)比较运算符
在MongoEngine中使用双下划线(__)分割。比如:age__gt=12
,表示年龄大于12的学生信息。
2)MongoEngine中的字符串查询 【i表示不区分大小写】
3)多个条件组合查询
- Q函数的使用:from mongoengine.queryset.visitor import Q
- 多个条件同时满足:Student.objects.filter(Q(key1=value1) & Q(key2=value2))
- 多个条件部分满足:Student.objects.filter(Q(key1=value1) | Q(key2=value2))
from mongoengine.queryset.visitor import Q from model import Student, Grade, SexEnum class LearnMongoDBEngine(object): def get_one_student(self): """查询一个学生的信息""" student_info = Student.objects.first() print(student_info, student_info.id, student_info.stu_name, student_info.address) return student_info def get_all_student(self): """查询所有学生的信息""" student_all_info = Student.objects.all() print(student_all_info) return student_all_info def get_student_by_id(self, pk: str): """根据学生的id查询""" student_info_id = Student.objects.get(id=pk) print(student_info_id) return student_info_id def get_student_1(self): """获取大于12岁的学生信息""" result = Student.objects.filter(age__gt=12) print(result) return result def get_student_2(self): """获取所有姓李的学生""" result = Student.objects.filter(stu_name__startswith='李') print(result) return result def get_student_3(self): """查询年龄在9~12之间(含)的学生信息""" # SELECT * FROM school_student_info WHERE age BETWEEN 9 AND 12; # db.students.find({'age': {'$gte': 9, '$lte': 12}}) result = Student.objects.filter(Q(age__gte=9) & Q(age__lte=12)) print(result) return result def get_student_4(self): """查询所有12岁以上的男生和9岁以下的女生""" result = Student.objects.filter(Q(age__gt=12, sex=SexEnum.MAN) | Q(age__lt=9, sex=SexEnum.WOMAN)) print(result) return result def get_grade(self): """查询大于等于60分的学生成绩信息""" result = Student.objects.filter(grades__score__gte=80) # 注意这儿写法 print(result) for i in result: print(i.grades) return result if __name__ == '__main__': obj = LearnMongoDBEngine() obj.get_one_student() obj.get_student_1() obj.get_student_2() obj.get_student_3() obj.get_student_4() obj.get_grade()
运行结果: 【注意:打印信息是由model中__call__函数设置决定的】
4.3、聚合统计
- 满足条件的文档数:
User.objects.count()
,所有的结果集都可以使用 - 求和/平均数:
User.objects.filter().sum(field) / User.objects.filter().average(field)
from mongoengine.queryset.visitor import Q from model import Student, Grade, SexEnum class LearnMongoDBEngine(object): def get_student_4(self): """查询所有12岁以上的男生和9岁以下的女生""" result = Student.objects.filter(Q(age__gt=12, sex=SexEnum.MAN) | Q(age__lt=9, sex=SexEnum.WOMAN)) print(result) return result def get_student_5(self): """查询所有12岁以上的男生和9岁以下的女生总数""" result = Student.objects.filter(Q(age__gt=12, sex=SexEnum.MAN) | Q(age__lt=9, sex=SexEnum.WOMAN)).count() print(result) return result def get_man_sex(self): """统计性别为男的总人数,并求出其平均年龄和总年龄""" queryset = Student.objects.filter(sex='男') print(queryset) man_num_count = queryset.count() print(man_num_count) man_avg_age = queryset.average('age') print(man_avg_age) man_sum_age = queryset.sum('age') print(man_sum_age) return man_avg_age, man_sum_age if __name__ == '__main__': obj = LearnMongoDBEngine() obj.get_student_4() obj.get_student_5() obj.get_man_sex()
运行结果:
4.4、排序
- -:倒叙排列
- (+):正序排序,默认就是正序
- Student.objects().order_by('field1', '-field2')
from mongoengine.queryset.visitor import Q from model import Student, Grade, SexEnum class LearnMongoDBEngine(object): def get_max_min_age(self): """获取最大年龄和最小年龄的学生信息""" queryset_min_age = Student.objects.order_by('age').first() queryset_max_age = Student.objects.order_by('-age').first() print(queryset_min_age, queryset_min_age.age) print(queryset_max_age, queryset_max_age.age) return '200 OK' if __name__ == '__main__': obj = LearnMongoDBEngine() obj.get_max_min_age()
运行结果:
4.5、分页处理
- 方式一,切片方式:
User.objects.all()[10:15]
- 方式二,.skip().limit():
User.objects.skip(10).limit(5)
from mongoengine.queryset.visitor import Q from model import Student, Grade, SexEnum class LearnMongoDBEngine(object): def paginate(self, page: int = 1, page_size: int = 5): """ 分页处理 :param page: 当前是第几页 :param page_size: 每页有多少数据 :return: """ # 方法一:切片 start = (page - 1) * page_size end = start + page_size queryset1 = Student.objects.all()[start:end] print(queryset1) # 方法二:skip().limit() queryset2 = Student.objects.skip(start).limit(page_size) print(queryset2) return "200 OK" if __name__ == '__main__': obj = LearnMongoDBEngine() obj.paginate(1, 6) print("*" * 100) obj.paginate(1, 3) obj.paginate(2, 3)
运行结果:
五、修改和删除数据
5.1、修改数据
修改数据时一般先过滤数据,再修改
- 修改一条数据:
User.objects.filter().update_one()
- 批量修改数据:
User.objects.filter().update()
from mongoengine.queryset.visitor import Q from model import Student, Grade, SexEnum class LearnMongoDBEngine(object): def update_one(self): """修改一条数据""" queryset = Student.objects.filter(stu_no='2438197') print(queryset) result = queryset.update_one(stu_name='白龙马', phone_no='16452412564') # result = queryset.update_one(stu_name='白龙马', unset__phone_no=True) print(result) def update_one_2(self): """修改一条数据""" queryset = Student.objects.filter(stu_no=3152784).first() print(queryset) if queryset: queryset.stu_name = '沙和尚' result = queryset.save() print(result) return "200 OK" else: return "error" def update_many(self): """将年龄10岁的学生年龄加一岁""" queryset = Student.objects.filter(age=10) print(queryset) queryset.update(inc__age=1) print(queryset) return "200 OK" if __name__ == '__main__': obj = LearnMongoDBEngine() obj.update_one() obj.update_one_2() obj.update_many()
运行结果:
5.2、删除数据
User.objects.filter().delete()
from mongoengine.queryset.visitor import Q from model import Student, Grade, SexEnum class LearnMongoDBEngine(object): def delete_data(self): """删除年龄大于13岁的学生""" queryset_start = Student.objects.all() print(f"删除前学生总数量:{queryset_start.count()}") queryset = Student.objects.filter(age__gt=13) print(f'删除的学生的数量:{queryset.count()}') res = queryset.delete() print(f"删除的结果:{res}") queryset_end = Student.objects.all() print(f"删除后剩余学生总数量:{queryset_end.count()}") if __name__ == '__main__': obj = LearnMongoDBEngine() obj.delete_data()
运行结果:
附录:
main.py
import random from mongoengine.queryset.visitor import Q from model import Student, Grade, SexEnum, CourseGrade class LearnMongoDBEngine(object): def __init__(self, info): self.info = info print(self.info) def add_one_student(self): """新增一个学生信息""" result = Student.objects.create( stu_no=random.randint(3000, 9999999), stu_name=self.info['name'], sex=random.choice([SexEnum.MAN, SexEnum.WOMAN]), class_name='六年级三班', address=self.info['address'], phone_no=self.info['phone'], age=random.randint(10, 15), grades=[ CourseGrade(course_name='语文', teacher=self.info['teacher'], score=random.randint(1, 100)), CourseGrade(course_name='数学', teacher=self.info['teacher'], score=random.randint(1, 100)), CourseGrade(course_name='英语', teacher=self.info['teacher'], score=random.randint(1, 100)), ] ) # student = Student( # stu_no=random.randint(3000, 9999999), # stu_name=self.info['name'], # sex=random.choice([SexEnum.MAN, SexEnum.WOMAN]), # class_name='六年级三班', # address=self.info['address'], # phone_no=self.info['phone'], # age=random.randint(10, 15), # grades=[ # CourseGrade(course_name='语文', teacher=self.info['teacher'], score=random.randint(1, 100)), # CourseGrade(course_name='数学', teacher=self.info['teacher'], score=random.randint(1, 100)), # CourseGrade(course_name='英语', teacher=self.info['teacher'], score=random.randint(1, 100)), # ] # ) print(result, result.grades) # result = student.save() return result def get_one_student(self): """查询一个学生的信息""" student_info = Student.objects.first() print(student_info, student_info.id, student_info.stu_name, student_info.address) return student_info def get_all_student(self): """查询所有学生的信息""" student_all_info = Student.objects.all() print(student_all_info) return student_all_info def get_student_by_id(self, pk: str): """根据学生的id查询""" student_info_id = Student.objects.get(id=pk) print(student_info_id) return student_info_id def get_student_1(self): """获取大于12岁的学生信息""" result = Student.objects.filter(age__gt=12) print(result) return result def get_student_2(self): """获取所有姓李的学生""" result = Student.objects.filter(stu_name__startswith='李') print(result) return result def get_student_3(self): """查询年龄在9~12之间(含)的学生信息""" # SELECT * FROM school_student_info WHERE age BETWEEN 9 AND 12; # db.students.find({'age': {'$gte': 9, '$lte': 12}}) result = Student.objects.filter(Q(age__gte=9) & Q(age__lte=12)) print(result) return result def get_student_4(self): """查询所有12岁以上的男生和9岁以下的女生""" result = Student.objects.filter(Q(age__gt=12, sex=SexEnum.MAN) | Q(age__lt=9, sex=SexEnum.WOMAN)) print(result) return result def get_student_5(self): """查询所有12岁以上的男生和9岁以下的女生总数""" result = Student.objects.filter(Q(age__gt=12, sex=SexEnum.MAN) | Q(age__lt=9, sex=SexEnum.WOMAN)).count() print(result) return result def get_man_sex(self): """统计性别为男的总人数,并求出其平均年龄和总年龄""" queryset = Student.objects.filter(sex='男') print(queryset) man_num_count = queryset.count() print(man_num_count) man_avg_age = queryset.average('age') print(man_avg_age) man_sum_age = queryset.sum('age') print(man_sum_age) return man_avg_age, man_sum_age def get_grade(self): """查询大于等于60分的学生成绩信息""" result = Student.objects.filter(grades__score__gte=80) # 注意这儿写法 print(result) for i in result: print(i.grades) return result def get_max_min_age(self): """获取最大年龄和最小年龄的学生信息""" queryset_min_age = Student.objects.order_by('age').first() queryset_max_age = Student.objects.order_by('-age').first() print(queryset_min_age, queryset_min_age.age) print(queryset_max_age, queryset_max_age.age) return '200 OK' def paginate(self, page: int = 1, page_size: int = 5): """ 分页处理 :param page: 当前是第几页 :param page_size: 每页有多少数据 :return: """ # 方法一:切片 start = (page - 1) * page_size end = start + page_size queryset1 = Student.objects.all()[start:end] print(queryset1) # 方法二:skip().limit() queryset2 = Student.objects.skip(start).limit(page_size) print(queryset2) return "200 OK" def update_one(self): """修改一条数据""" queryset = Student.objects.filter(stu_no='2438197') print(queryset) result = queryset.update_one(stu_name='白龙马', phone_no='16452412564') # result = queryset.update_one(stu_name='白龙马', unset__phone_no=True) print(result) def update_one_2(self): """修改一条数据""" queryset = Student.objects.filter(stu_no=3152784).first() print(queryset) if queryset: queryset.stu_name = '沙和尚' result = queryset.save() print(result) return "200 OK" else: return "error" def update_many(self): """将年龄10岁的学生年龄加一岁""" queryset = Student.objects.filter(age=10) print(queryset) queryset.update(inc__age=1) print(queryset) return "200 OK" def delete_data(self): """删除年龄大于13岁的学生""" queryset_start = Student.objects.all() print(f"删除前学生总数量:{queryset_start.count()}") queryset = Student.objects.filter(age__gt=13) print(f'删除的学生的数量:{queryset.count()}') res = queryset.delete() print(f"删除的结果:{res}") queryset_end = Student.objects.all() print(f"删除后剩余学生总数量:{queryset_end.count()}") if __name__ == '__main__': # 自我测试代码 pass
model.py
from mongoengine import Document, connect, EnumField, StringField, IntField, ListField, EmbeddedDocument, \ EmbeddedDocumentField from enum import Enum # 连接到User数据库 connect('user', host='192.168.124.104', port=27017) class SexEnum(Enum): MAN = '男' WOMAN = '女' class CourseGrade(EmbeddedDocument): """成绩信息(科目、老师、成绩)-被嵌套的文档""" course_name = StringField(max_length=64, required=True, verbose_name='科目') teacher = StringField(max_length=16, verbose_name='老师') score = IntField(min_value=0, max_value=150, required=True, verbose_name='成绩') def __repr__(self): return f"CourseGrade({self.course_name},{self.score})" def __str__(self): return self.__repr__() class Student(Document): """学生信息""" # verbose_name 自定义参数,用于显示定义域的名称 stu_no = IntField(required=True, unique=True, verbose_name='学号') stu_name = StringField(required=True, max_length=16, verbose_name='姓名') sex = EnumField(enum=SexEnum, verbose_name='性别') class_name = StringField(max_length=10, verbose_name='班级') address = StringField(max_length=255, verbose_name='家庭住址') phone_no = StringField(max_length=11, verbose_name='电话号码') age = IntField(min_value=0, max_value=150, verbose_name='年龄') grades = ListField(EmbeddedDocumentField(CourseGrade), verbose_name='成绩数组') meta = { # 指定文档的集合 'collection': 'students', # 指定排序,可以指定多个域。例如:'age':根据年龄升序,'-age':根据年龄降序 'ordering': ['-age'], 'strict': False # 设置非严格校验字段则不需要吧所有字段都声明 } def __repr__(self): return f'Grade({self.stu_no}, {self.stu_name})' def __str__(self): return self.__repr__() class Grade(Document): """学生成绩""" # verbose_name 自定义参数,用于显示定义域的名称 stu_no = IntField(required=True, verbose_name="学号") stu_name = StringField(required=True, max_length=16, verbose_name='姓名') sex = EnumField(enum=SexEnum, verbose_name='性别') class_name = StringField(max_length=10, verbose_name='班级') address = StringField(max_length=255, verbose_name='家庭住址') phone_no = StringField(max_length=11, verbose_name='电话号码') age = IntField(min_value=0, max_value=150, verbose_name='年龄') grades = EmbeddedDocumentField(CourseGrade, verbose_name='成绩') meta = { # 指定文档的集合 'collection': 'grades', # 指定排序,可以指定多个域。例如:'age':根据年龄升序,'-age':根据年龄降序 'ordering': ['-age'] } def __repr__(self): return f'Grade({self.stu_no}, {self.stu_name})' def __str__(self): return self.__repr__()
官方文档:
https://docs.mongoengine.org/guide/querying.html
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