{"category":"animal","category_id":"1"}
 {"category":"fruit","category_id":"2"}
 {"category":"sport","category_id":"3"}

{"id":"1","name":"apple","category":"fruit"}
 {"id":"2","name":"pear","category":"fruit"}
 {"id":"3","name":"football","category":"sport"}
 {"id":"4","name":"basketball","category":"sport"}
 {"id":"5","name":"cat","category":"animal"}
 {"id":"6","name":"dog","category":"animal"}

This is a reference problem between collections. The solution code is as follows.

fruit=db.categories.findOne({"category":"fruit"})
 db.details.updateMany({"category":"fruit"},{$set:{"category_id":fruit.categsory_id}})

The above code adds the category_id item to all {“category”:”fruit”} items in details, and its value is the category_id value corresponding to {“category”:”fruit”} items in categories.

Adding category_id to the rest of details is the same, just replace the part of fruit with animal and sport.

Cursor upgrade …

db.details.find().forEach( (x)=> {
 db.categories.find().forEach( (y)=> {
 if(y.category===x.category){
 db.details.updateMany({"category":x.category},{$set:{"category_id":y.category_id}})
 }
 })
 })