mongodb中的字符串排序

前言

数据库有一索引字段，ObjectId与string类型混用。
需要辨明sort后的排序关系。

输入

[{
  "_id": {
    "$oid": "e98799a7e181225ad5c87476"
  }
},
{
  "_id": {
    "$oid": "e98799a7e181225ad5c87477"
  }
},
{
  "_id": {
    "$oid": "e98799a7e181225ad5c87480"
  }
},
{
  "_id": "e98799a7e181225ad5c87476"
},
{
  "_id": "e98799a7e181225ad5c87476ccd7fc61"
},
{
  "_id": "e98799a7e181225ad5c87478"
},
{
  "_id": "e98799a7e181225ad5c87479"
}]

结论1 混排

经过测试，ObjectId与string类型混用时，
使用Ascending排序，ObjectId总是排在string之后。
string内部遵循字典序。

for doc in coll.find({}).sort("_id", pymongo.ASCENDING):
    print(doc)
"""
{'_id': 'e98799a7e181225ad5c87476'}
{'_id': 'e98799a7e181225ad5c87476ccd7fc61'}
{'_id': 'e98799a7e181225ad5c87478'}
{'_id': 'e98799a7e181225ad5c87479'}
{'_id': ObjectId('e98799a7e181225ad5c87476')}
{'_id': ObjectId('e98799a7e181225ad5c87477')}
{'_id': ObjectId('e98799a7e181225ad5c87480')}
"""

结论2 条件比较

在mongo内部，给定string作为比较条件时，只会和string类型直接进行比较。
返回值中不再出现ObjectId类型。

for doc in coll.find({'_id':{'$gt':'e98799a7e181225ad5c87476'}}).sort("_id", pymongo.ASCENDING):
    print(doc)
"""
{'_id': 'e98799a7e181225ad5c87476ccd7fc61'}
{'_id': 'e98799a7e181225ad5c87478'}
{'_id': 'e98799a7e181225ad5c87479'}
"""

类似地，给定ObjectId作为比较对象时，返回值中也只存在ObjectId类型。

for doc in coll.find({'_id':{'$gt': ObjectId('e98799a7e181225ad5c87476')}}).sort("_id", pymongo.ASCENDING):
    print(doc)

"""
{'_id': ObjectId('e98799a7e181225ad5c87477')}
{'_id': ObjectId('e98799a7e181225ad5c87480')}
"""

想选中指定类型只需要

for doc in coll.find({'_id':{'$type':'objectId'}}).sort("_id", pymongo.ASCENDING):
    print(doc)

"""
{'_id': ObjectId('e98799a7e181225ad5c87476')}
{'_id': ObjectId('e98799a7e181225ad5c87477')}
{'_id': ObjectId('e98799a7e181225ad5c87480')}
"""
# or
for doc in coll.find({'_id':{'$type':'string'}}).sort("_id", pymongo.ASCENDING):
    print(doc)

"""
{'_id': 'e98799a7e181225ad5c87476'}
{'_id': 'e98799a7e181225ad5c87476ccd7fc61'}
{'_id': 'e98799a7e181225ad5c87478'}
{'_id': 'e98799a7e181225ad5c87479'}
"""

联合查询

准确地使用 类型指定$type + 比较条件$gt时可以获得返回值。
但如果$type给定string，$gt给定ObjectId就会返回空。

query = { '$and':[{'_id': {'$type': "string"}}, {'_id':{'$gt':'e98'}}]}

"""
{'_id': 'e98799a7e181225ad5c87476'}
{'_id': 'e98799a7e181225ad5c87476ccd7fc61'}
{'_id': 'e98799a7e181225ad5c87478'}
{'_id': 'e98799a7e181225ad5c87479'}
"""

# or
for doc in coll.find({ '$and':[{'_id': {'$type': "objectId"}}, {'_id':{'$gt':ObjectId('e98799a7e181225ad5c87476')}}]}).sort("_id", pymongo.ASCENDING):
    print(doc)

"""
{'_id': ObjectId('e98799a7e181225ad5c87477')}
{'_id': ObjectId('e98799a7e181225ad5c87480')}
"""