如何限制从MongoDB查询中的字段返回的字符数？
为此，请使用MongoDB$substr。首先让我们创建一个包含文档的集合-
> dblimitTheNumberOfCharactersDemoinsertOne({"Title":"MongoDB is No SQL database"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cf23013b64a577be5a2bc0e")
}
> dblimitTheNumberOfCharactersDemoinsertOne({"Title":"MySQL is a relational database"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cf2302db64a577be5a2bc0f")
}以下是在find()方法的帮助下显示集合中所有文档的查询-
> dblimitTheNumberOfCharactersDemofind()pretty();
这将产生以下文件-
{
"_id" : ObjectId("5cf23013b64a577be5a2bc0e"),
"Title" : "MongoDB is No SQL database"
}
{
"_id" : ObjectId("5cf2302db64a577be5a2bc0f"),
"Title" : "MySQL is a relational database"
}以下是限制从MongoDB查询中的字段返回的字符数量的查询-
> dblimitTheNumberOfCharactersDemoaggregate(
[
{
$project:
{
Characters: { $substr: [ "$Title", 0, 20 ] }
}
}
]
);这将产生以下文件-
{ "_id" : ObjectId("5cf23013b64a577be5a2bc0e"), "Characters" : "MongoDB is No SQL da" }
{ "_id" : ObjectId("5cf2302db64a577be5a2bc0f"), "Characters" : "MySQL is a relationa" }