在MongoDB和golang（mgo）中需要在$ push中包含$ each和$ position

Question

1.In the back end i m using go lang and for database i use mongoDB. I m trying to find the last document inserted in the embedded array so i can retrieve the document in the last array index without knowing its index.Is is possible?? After researching on this i came to know that its not possible.So i m thinking of using $push,$each and $position.Here i can set the position to 0 so the newly added document will be in 0 so i can retreive it using the index 0.

  Here is bson format
    {
    empid:"L12"
    AnnualLeave:[
        {
        "atotal" : 20,
        }
    ]
    }



 Here is my schema


 type (
            Employee struct {
                EmpId          string
                AnnualLeave    []*AnnualLeaveInfo
            }
             AnnualLeaveInfo struct {
                ATotal          int64   
            }


        I use the mgo statement as follows`enter code here`

 c.Update(bson.M{"empid": "string"}, bson.M{"$push": bson.M{"annualleave":bson.M{"$each":
         bson.M{"atotal": 4},"$position":0}}

2.Please advice me as well how to decrement the ATotal of the previous document attached and keep it as the value of the atotal of the new document. Please help me.Thanks

Answer 1

I m trying to find the last document inserted in the embedded array so i can retrieve the document in the last array index without knowing its index.Is is possible?

You can find the last array index by derriving from the array length. Using your example:

type Employee struct {
        EmpId string 
        AnnualLeave []AnnualLeaveInfo
}
type AnnualLeaveInfo struct {
        ATotal int64
}

result := Employee{}
err = c.Find(bson.M{"empid": "example employee ID"}).One(&result)
if err != nil {
        log.Fatal(err)
}
lastAnnualTotal:= result.AnnualLeave[len(result.AnnualLeave)-1].ATotal

Please advice me as well how to decrement the ATotal of the previous document attached and keep it as the value of the atotal of the new document

Depending on your use case, you could try performing two database operations:

• Fetch the last ATotal value from the collection.
• Push a new AnnualLeaveInfo document with the new ATotal value.

// Assuming that EmpId is unique
err = c.Update(bson.M{"empid": result.EmpId},
      bson.M{"$push": bson.M{"annualleave": bson.M{"atotal": int(latestAnnualTotal-1)}}})

If you require atomic updates, see MongoDB Atomicity and Transactions and Model Data for Atomic Operations.

On another note, it seems that you're trying to do something related to CQRS design patterns. This design pattern may help with calculating your annual leave use case. See also Even Sourcing with MongoDB