Mongoose condition queries part of the data of the specified attribute

I encountered a problem of data query when I was rolling out the code today , How to retrieve some specified values of an attribute in a database table , for example , Data sheet stable There is an attribute in it, which is status, Its value may be a,b,c,d, Now I just need to retrieve all the items in the table status The value of is a,b The data of , How to achieve ？

Previously, I wrote the function of retrieving some specified attributes in the database , Here is how to realize this requirement , Use mongoose Of model Of select Method , You can retrieve the required data , There are two ways of writing ：

• model.select(“item1 item2 item3”);

stay select The attribute name to be retrieved is passed in the method , Just separate it with a space , This kind of writing is more direct , If you need to retrieve item1 Child attributes of a, You can write directly item1.a That's all right. , Such as model.select(“item1.a item2 item3”);

• model.select({item: 1, item2: 1, item: 0});

stay select Method to pass in an object ,1 Represents that this attribute needs to be retrieved ,0 Representing this attribute does not need to be retrieved , Properties that do not need to be retrieved do not need to be written .
These two ways of writing can be achieved , Depending on personal preference .

So how to realize the problem of the beginning of the article ,mongoose There is an interesting way to write ,“$”, It can realize some interesting functions .

• $ in Can solve this problem , It means that one key corresponds to multiple values , for example model.find({status: {$in:[a,b]}}); What it does is retrieve all status The value of is a,b The data of .

• $ nin We can also solve this problem , It means that a key does not correspond to the specified value , for example model.find({status: {$nin:[c,d]}}); It implements the retrieval of all status The value of the c,d The data of .

The two writing methods are applicable to different scenes , Their realizable functions are also different .

I also sorted out some other ways of writing , You can have a look if you need .

$lt  Less than a certain value  {age: {$lt: 30}}
$lte  Less than or equal to a certain value  {age: {$lte: 30}}
$gt  Greater than a certain value  {age: {$gt: 30}}
$gte  Greater than or equal to  {age: {$gte: 30}}
$eq  be equal to  {age: {$eq: 30}}
$ne  It's not equal to  {age: { $ne:24}}
$in  A key corresponds to multiple values  {age: {$in:[20,30]}}
$nin  A key does not correspond to the specified value  {age: {$nin:[20,30]}}
$or  Multiple condition matching ,  Can be nested  $in  Use   {$or: [{name: "name"}, {age: 28}]}
$nor  Query documents that do not match a particular pattern 
$exists  Determine whether some key fields exist  {name: {$exists: true}}