How to update particular value inside array - Mongodb

Question

I have a mongodb collection Users that looks like this:

{
 "_id" : ObjectId("5dba8987b4a39c13bc104a23"),
 "contact" : {
 "firstName" : "Mark",
 "lastName" : "Doe",
 "parentsBloodType" : [ 
 {
 "type" : "AB+",
 }, 
 {
 "type" : "A+",
 }, 
 ],
 },
 "createdAt" : ISODate("2019-10-31T07:13:11.278Z"),
 "updatedAt" : ISODate("2019-11-26T09:59:41.611Z")
}

I need to run a raw query to update from AB+ to O-. I would also like to check if they match before updating.

I tried this but it added an extra field to the User:

 db.getCollection('users').update(
 {"_id": ObjectId("5dba8987b4a39c13bc104a23")},
 {"$set" : { 'parentsBloodType.' + '0' + '. type' : "O-"}}
 )

• Wernfried Domscheit: What happens if both parents have AB+? Do you like to update both or just one?
• Tom Bom: Thanks for pointing it out, I would like to update both.

Answer 1

This one would do the update:

db.users.update(
 { "_id": ObjectId("5dba8987b4a39c13bc104a23") },
 { "$set": { 'contact.parentsBloodType.0.type' : "O-"} }
)

You an check match with this:

db.users.update(
 {
 "_id": ObjectId("5dba8987b4a39c13bc104a23"),
 "contact.parentsBloodType.type": "AB+"
 },
 { "$set": { 'contact.parentsBloodType.0.type': "AB+" } }
)

This updates the document only if type AB+ exist (at any parent).

Or if you like to check whether the first type is AB+ then use

db.users.update(
 {
 "_id": ObjectId("5dba8987b4a39c13bc104a23"),
 "contact.parentsBloodType.0.type": "AB+"
 },
 { "$set": { 'contact.parentsBloodType.0.type': "AB+" } }
)

However, I assume you are actually looking for this:

db.users.update(
 { "_id": ObjectId("5dba8987b4a39c13bc104a23") },
 { "$set": { 'contact.parentsBloodType.$[p].type': "0-" } },
 { arrayFilters: [{ "p.type": "AB+" }] }
)

Which will update any AB+ to 0- no matter on which position it appears in the array.

Answer 2

You could use the positional $ operator to update the first element that matches the query document:

db.getCollection('users').update(
 { "_id": ObjectId("5dba8987b4a39c13bc104a23"), "parentsBloodType.type": "AB+" },
 { "$set" : { 'parentsBloodType.$.type' : "O-" } }
)

note that:

• the positional $ operator acts as a placeholder for the first element that matches the query document, and
• the array field must appear as part of the query document.

Or use the filtered positional operator $[] if you want to update all elements that match an array filter condition or conditions:

db.students.update(
 { "_id": ObjectId("5dba8987b4a39c13bc104a23") },
 { $set: { "parentsBloodType.$[element].type" : "O-" } },
 { 
 arrayFilters: [ { "element.type": "AB+" } ]
 }
)

• Wernfried Domscheit: Just a note, multi: true means update multiple documents - not update multiple values of an array. I.e. regardless of multi: true or multi: false it will update all matching values in the array. Acctually when you filter on primary key _id: ObjectId(...) then multi: true is pointless.