MongoDB--add/drop elements in a field of array of hash how to

Question

I am using perl MongoDBx::Class, following the tutorial I inserted the document below. The tags field is an array of hash. Tried to remove tag and add tag using mongodb's $pull and $addToSet function without success.

How to add/drop elements to/from tags field? If you are not a perl programmer, answer in mongodb shell command also wlcome.

Thanks.

my $novel = $novels_coll->insert({
   _class => 'Novel',
   title => 'The Valley of Fear',
   year => 1914,
   author => {
      first_name => 'Arthur',
      middle_name => 'Conan',
      last_name => 'Doyle',
   },
   added => DateTime->now(time_zone => 'Asia/Jerusalem'),
   tags => [
      { category => 'mystery', subcategory => 'thriller' },
      { category => 'mystery', subcategory => 'detective' },
      { category => 'crime', subcategory => 'fiction' },
   ],
});

This is the document inserted:

{
    "_id": {
        "$oid": "4e27eae3008a6ee40f000000"
    },
    "_class": "Novel",
    "added": "2011-07-21T12:01:23+03:00",
    "author": {
        "middle_name": "Conan",
        "last_name": "Doyle",
        "first_name": "Arthur"
    },
    "tags": [
        {
            "subcategory": "thriller",
            "category": "mystery"
        },
        {
            "subcategory": "detective",
            "category": "mystery"
        },
        {
            "subcategory": "fiction",
            "category": "crime"
        }
    ],
    "title": "The Valley of Fear",
    "year": 1914
}

Edit:

After deeper exploration of MongoDBX, the update method overrided offical MongoDB driver, so the $pull, $addToSet may not work. I will use this stupid method:

my $novel = $novel_coll->find_one({ some criteria });
my @tags = $novel->tags;
my @updated_tags = grep(unless($tag{category=>CATEGORY}, @tags);  #to pull out unwanted tag. Or use push to add new tag.
$novel->update({tags=>\@updated_tags});

I hope MongoDBx has method to updated arrayRef field.

Remon van Vliet · Accepted Answer · 2011-07-22 12:08:03Z

1

I'll do this in mongo shell syntax, convert to perl as needed :

ADD :

db.novels_coll.update({_id: ID, ...other criteria...}, {$addToSet:{tags:{subcategory:SUBCATEGORY, category:CATEGORY}}})

REMOVE BY CATEGORY :

db.novels_coll.update({_id: ID, ...other criteria...}, {$pull:{tags:{category:CATEGORY}}})

REMOVE BY CATEGORY AND SUBCATEGORY :

db.novels_coll.update({_id: ID, ...other criteria...}, {$pull:{tags:{category:CATEGORY, subCategory: SUBCATEGORY}}})

Does that answer your question?

answered Jul 22, 2011 at 12:08

Remon van Vliet

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1 Comment

Weiyan Over a year ago

Thank you. If I use official MongoDB, I had use $pull and $addToSet in an array, but don't know how to use in array of hash. MongoDBx::Class have update method that avoid the $set as per offcial MongoDB api. Having explore for one night, I am thinking another stupid solution, added to orginal post as edit.

ido50 · Accepted Answer · 2011-08-02 12:17:29Z

I am the author of MongoDBx::Class.

When using MongoDBx::Class, you have two options to update a document:

The regular, MongoDB way, by using the update() method on the collection object:

$collection->update({ _id => $document->_id }, { '$addToSet' => { tags => { category => 'drama', subcategory => 'chick-flick' } } });

This allows you to perform all of MongoDB's modifiers such as $addToSet, $push, $pull, etc.

The MongoDBx::Class way, by using the update() method on the document object:

$document->update({ tags => \@new_tags });

This is faster and somewhat cleaner, but it only really performs the $set modifier, so whatever you provide to the "tags" attribute in the example above will be the new value of that attribute.

Hope this helps, Ido.

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