如何限制 MongoDB 中某个字段返回的字符数?
mongodbdatabasebig data analytics
要限制某个字段返回的字符数，可以使用 MongoDB 中的 `$substr` 函数。让我们创建一个包含文档的集合。
> db.demo233.insertOne({"Paragraph":"My Name is John Smith.I am learning MongoDB database"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e41877df4cebbeaebec5146")
}
> db.demo233.insertOne({"Paragraph":"David Miller is a good student and learning Spring and Hibernate Framework."});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e4187d7f4cebbeaebec5147")
}
借助 find() 方法显示集合中的所有文档 −
> db.demo233.find().pretty();
这将产生以下输出 −
{
"_id" : ObjectId("5e41877df4cebbeaebec5146"),
"Paragraph" : "My Name is John Smith.I am learning MongoDB database"
}
{
"_id" : ObjectId("5e4187d7f4cebbeaebec5147"),
"Paragraph" : "David Miller is a good student and learning Spring and Hibernate Framework."
}
以下查询用于限制 MongoDB 中某个字段返回的字符数 −
> db.demo233.aggregate(
... [
... {
... $project:
... {
... Paragraph: { $substr: [ "$Paragraph", 0, 10] }
...
... }
...} ] )
这将产生以下输出 −
{ "_id" : ObjectId("5e41877df4cebbeaebec5146"), "Paragraph" : "My Name is" }
{ "_id" : ObjectId("5e4187d7f4cebbeaebec5147"), "Paragraph" : "David Mill" }
