Mongocxx - Update an array within a document?

Question

I have a document in MongoDB with the following structure:

{
"_id":"$oid":"621fbaeaeedd1c000e60fbd2"},
"username":"myuser",
"password":"mypassword",
"comments":["comment1", "comment2", "comment3"]
}

I have a vector of comments:

std::vector<std::string> mycomments; 
mycomments.push_back("comment2"); 
mycomments.push_back("comment4");

I would like to insert, if it is not already present, each string of the vector "mycomments" into the array "comments" in MongoDB document. I read MongoDB documentation, but I did not understand how to use the function "update_one", as in the documentation there is only a very simple case. So the solution that I adopted is the following: I get the content of comments and I put it into a vector, I push each string of mycomments into this vector (if it is not already present), then I delete the document in MongoDB and I insert a new document with new values:

bsoncxx::stdx::optional<bsoncxx::document::value> res = collection.find_one(document{} << "username" << username << finalize);
    if (res)
    {
        coll.delete_one(document{} << "username" << username << finalize);

        document data_builder{};
        data_builder << "username" << username << "password" << password;
        auto array_builder = data_builder << "comments" << open_array;
        for (std::string str : myNewVector)
        {
            array_builder << str;
        }
        array_builder << close_array;
        bsoncxx::document::view_or_value doc = data_builder << finalize;
        coll.insert_one(doc);

    }

Clearly this is a very foolish solution, since it would be enough to use the update_one function, but from the documentation I cannot understand how to use it (in this complex case). Can you help me please?

Octo Poulos · Answer 1 · 2022-03-11T11:08:00.990

If you want to keep the order, then you must get the document and only push the comments that are not already present in it.

Then, you can use the $push and $each modifier. Here's the reference: https://docs.mongodb.com/manual/reference/operator/update/push/#append-multiple-values-to-an-array

I ran this code:

    std::vector<std::string> mycomments;
    mycomments.push_back("comment2");
    mycomments.push_back("comment4");

    auto eachDoc = document{};
    auto arrayDoc = eachDoc << "$each" << open_array;
    for (auto& comment : mycomments)
        arrayDoc << comment;
    arrayDoc << close_array;

    auto findDoc = document{} << "username" << "myuser" << finalize;
    auto update = document{} << "$push" << open_document << "comments" << eachDoc << close_document << finalize;

    collection.update_one(findDoc.view(), update.view());

And I obtained: ["comment1", "comment2", "comment3", "comment2", "comment4"]

score 0 · Answer 2 · answered Mar 04 '22 at 22:45

One way to add multiple strings to an array without duplicates is with the $addToSet operator and the $each modifier:

coll.update_one(
  make_document( kvp("username", "myuser") ), // filter part

  make_document(                              // update part
    kvp("$addToSet",
        make_document( kvp("comments",
                           make_document( kvp("$each",
                                              make_array("comment2", "comment4")))))))
);

score 0 · Answer 3 · answered Feb 06 '23 at 18:47

If you want to push new elements without tampering the existing array of strings,try the following code segment:

 bsoncxx::builder::basic::document update_builder;
    
/*Define the update to be performed on the document*/

bsoncxx::builder::basic::array new_array;
new_array.append("T1");
new_array.append("T2");
new_array.append("T3");
update_builder.append(kvp("$set", make_document(kvp("field_name", new_array))));
          
auto updating = update_builder.extract();
    
/*Update the document*/
auto result = m_dbCollection.update_one(filter.view(), updating.view());

For clearing the existing elements in the field and update it with new array of strings,you may try the following

/*Clear the existing data in the field*/

bsoncxx::builder::basic::document update_builder;
update_builder.append(kvp("$set",make_document(kvp("field_name", bsoncxx::types::b_array{}))));
auto update = update_builder.extract();
    
/*Define the update to be performed on the document*/

bsoncxx::builder::basic::array new_array;
new_array.append("T1");
new_array.append("T2");
new_array.append("T3");
update_builder.append(kvp("$set", make_document(kvp("field_name", new_array))));
          
auto updating = update_builder.extract();
    
/*Update the document*/
auto result = m_dbCollection.update_one(filter.view(), updating.view());

Mongocxx - Update an array within a document?

3 Answers3