Upsert Array Elements matching criteria in a Mongo

问题:

As per How do I update Array Elements matching criteria in a MongoDB document?

I want to upsert the array elements, so if one doesnt match then insert it, otherwise update it.

I tried the answer on that question, and it works fine IF the array element already exists. If the element doesnt exist then it creates a child of "$" under the array field.

My Mongo structure is as follows:

Widget (collection)
--Name
--Properties (array)
  --Name
  --Value

My application gets a Widget Name and a list of Properties from a WebService call. I wish to iterate the provided Properties and update the value in the MongoDB if the Name already exists, OR insert a new Property to the Properties array if it doesnt.

回答1:

What you require is not possible using a single update without some app-side logic. Note that upsert as a feature is not relevant for this specific problem unless you want to automatically create new Widget documents if none exist with the provided name.

The problem you're running into is that there is no functionality that allows you to do two different updates depending on the existence of an array element. Your only two options are :

1. Find the item, determine existence of relevant propertie(s), compile an appropriate update with your new or changes properties and execute it. This comes with the important downside that this is not a concurrency safe method. In other words, if two web services attempt this at the same one could overwrite eachother's changes.
2. Make widget properties top level documents rather than embedded. Allows you to use upserts to do what you want. Obvious downside is that that isn't a very nice option in terms of schema design. You wouldn't automatically get all properties if you fetch a widget, for example.

回答2:

You cannot atomically upsert array elements. But if you can restructure your document to use an object instead of an array, then this is atomically possible. Using your notation, the structure would be

Widget (collection)
  --Name
  --Properties
    --Property1
    --Property2
        .
        :

An example document would be

{
    name: 'widget-1',
    properties: {
        'property-1': 100,
        'property-2': 200
    }
}

To upsert an item named 'property-3' with value 300, you'd do

db.widgets.update(
    { name: 'widget-1' }, 
    { $set: { 'properties.property-3': 300 } }
)

One drawback is that querying for field names (using the $exists query operator) requires scanning. You can get around this by adding an extra array field that stores the property names. This field can be indexed and queried normally. Upserts become

db.widgets.update(
    { name: 'widget-1' }, 
    { 
        $set: { 'properties.property-3': 300 },
        $addToSet: { propertyNames: 'property-3' }
    }
)

(Keep in mind $addToSet is O(n).) When removing a property, you have to pull it from the array as well.

db.widgets.update(
    { name: 'widget-1' }, 
    { 
        $unset: { 'properties.property-3': true },
        $pull: { propertyNames: 'property-3' }
    }
)

回答3:

At least in php it works for me, try to adopt to your language. You need your collection look like:

        {Name: x, Properties: {name1:value1, name2:value2, name3:value3}}

        foreach ($Properties as $name => $value)
        {
            $propertiesArray["Properties.$name"] = $value;                
        }   

        $criteria  = array('Name' => $name);
        $operation = array('$set'   => $propertiesArray);
        $options   = array('upsert' => true);

        $collection = $this->_dbh->selectCollection('Widget');
        $collection->update($criteria, $operation, $options);

It inserts new names if not exist and updates those which already there