Insert or Update specific field

Requirement

We want to insert a new document if none of the documents match provided filter, but update only specific field when a match is found.

Current state of favorite_books collection

{  "_id": {    "$oid": "6294228d3811a7c4c5044808"  },  "title": "My Brilliant Friend",  "author": "Elena Ferrante",  "year_published": 2012}
{  "_id": {    "$oid": "6294228d3811a7c4c5044809"  },  "title": "Lucy",  "author": "Jamaica Kincaid",  "year_published": 2002}
{  "_id": {    "$oid": "6294228d3811a7c4c504480a"  },  "title": "Cat's Cradle",  "author": "Kurt Vonnegut Jr.",  "year_published": 1998}

Connect to database

const uri = "mongodb://localhost:27017/?maxPoolSize=20&w=majority"
client, err := mongo.Connect(context.TODO(), options.Client().ApplyURI(uri))

if err != nil {
    panic(err)
}
defer func() {
    if err = client.Disconnect(context.TODO()); err != nil {
        panic(err)
    }
}()

Insert or Update

coll := client.Database("myDB").Collection("favorite_books")
filter := bson.D{
    {"title", "Lucy"},
    {"year_published", 1866},
}
opts := options.Update().SetUpsert(true)
updateData := bson.D{{"title", "Crime and Punishment"}, {"year_published", 1866}}
update := bson.D{{"$set", bson.D{{"author", "Fyodor Dostoyevsky"}}}, {"$setOnInsert", updateData}}
res, err := coll.UpdateOne(context.TODO(), filter, update, opts)

if err != nil {
    fmt.Println(err)
}

if res.MatchedCount != 0 {
    fmt.Println("matched and updated an existing document")
}

if res.UpsertedCount != 0 {
    fmt.Println("inserted a document with id: %v\n", res.UpsertedID)
}

From the mongodb docs,

An upsert performs one of the following actions:

• Update documents that match your query filter
• Insert a document if there are no matches to your query filter

There is no document where title is Lucy and year_published is 1866, so the above code inserts a new document and the state of the collection changes to:

{  "_id": {    "$oid": "6294228d3811a7c4c5044808"  },  "title": "My Brilliant Friend",  "author": "Elena Ferrante",  "year_published": 2012}
{  "_id": {    "$oid": "6294228d3811a7c4c5044809"  },  "title": "Lucy",  "author": "Jamaica Kincaid",  "year_published": 2002}
{  "_id": {    "$oid": "6294228d3811a7c4c504480a"  },  "title": "Cat's Cradle",  "author": "Kurt Vonnegut Jr.",  "year_published": 1998}
{  "_id": {    "$oid": "629424222932d1cc570ad0d0"  },  "title": "Crime and Punishment",  "year_published": 1866,  "author": "Fyodor Dostoyevsky"}

If we change the filter to

filter := bson.D{
    {"title", "Crime and Punishment"},
    {"year_published", 1866},
}

there is a document that match this query filter, hence we can update a specific field with the line

update := bson.D{{"$set", bson.D{{"author", "Sasaki Kojirou"}}}, {"$setOnInsert", updateData}}

We then get the following collection:

{  "_id": {    "$oid": "6294228d3811a7c4c5044808"  },  "title": "My Brilliant Friend",  "author": "Elena Ferrante",  "year_published": 2012}
{  "_id": {    "$oid": "6294228d3811a7c4c5044809"  },  "title": "Lucy",  "author": "Jamaica Kincaid",  "year_published": 2002}
{  "_id": {    "$oid": "6294228d3811a7c4c504480a"  },  "title": "Cat's Cradle",  "author": "Kurt Vonnegut Jr.",  "year_published": 1998}
{  "_id": {    "$oid": "629424222932d1cc570ad0d0"  },  "title": "Crime and Punishment",  "year_published": 1866,  "author": "Sasaki Kojirou"}

Note: Beware that field operators $set and $setOnInsert shouldn't have same fields since it would throw write exception.

Reference:

Insert or Update in a single operation
setOnInsert