I have the following MongoDB data model:

```js
{
"_id" : ObjectId("53725814740fd6d2ee0ca2bb"),
"date" : "2014-01-01",
"establishmentId" : 1,
"products" : [
    {
        "productId" : 1,
        "price" : 7.03,
        "someOtherInfo" : 325,
        "somethingElse" : 6878
    },
    {
        "productId" : 2,
        "price" : 4.6,
        "someOtherInfo" : 243,
        "somethingElse" : 1757
    },
    {
        "productId" : 3,
        "price" : 2.14,
        "someOtherInfo" : 610,
        "somethingElse" : 5435
    },
    {
        "productId" : 4,
        "price" : 1.45,
        "someOtherInfo" : 627,
        "somethingElse" : 5762
    },
    {
        "productId" : 5,
        "price" : 3.9,
        "someOtherInfo" : 989,
        "somethingElse" : 3752
    }
}

```

What is the fastest way to get the average price across all establishments? Is there a better data model to achieve this?