Here are a few queries and miscellaneous commands that I like to keep on hand, and may help you as well.

MongoDB Cheat Sheet

Restore a multi-collection Mongo DB dump.

tar -xzvf /tmp/backup.tar.gz -C /tmp && mongorestore --drop /tmp/databaseName/

Export data from a Mongo collection to a CSV.

mongoexport -u USERNAME -p PASSWORD -c "COLLECTION" -h HOST -d DB -q '{whateverYourQueryIs: true}' -f '_id,field1,field2,field3' --csv -o output.csv

Query by and sort by date.

db.somecollection.find({someDateField: { $gte: new Date('2015-01-01') }}).sort({someDateField: 1}).limit(1).pretty()

Find documents containing a non-empty object.

db.somecollection.find({someObjectField: {$exists: true, $gt: {}}}).limit(1).pretty()

Add an object to an array in a document.

// This assumes the array already exists on the document. We are simply adding an item to it.
db.somecollection.update({someCriteria: 'whatever'}, {$push: {"someArray": "someValue"}})

Add an object to an array nested in an object on a document.

// Document looks like: { _id: 123, someObject: { someArray: [] } }
db.somecollection.update({someCriteria: 'whatever'}, {$push: {"someObject.someArray": "someValue"}})

Remove an object from an array nested in on object on a document.

// Document looks like: { _id: 123, someObject: { someArray: ['someValue'] } }
// Note the use of `$pull` instead of `$push`.
db.somecollection.update({someCriteria: 'whatever'}, {$pull: {"someObject.someArray": "someValue"}})

Find documents by array length.

// This would find any document where `someArray` has an item at position `0`, and so has at least one item.
db.somecollection.find({'someArray.0': {$exists: true}})

Aggregate documents to find patterns in our data.

// In this example, we query for a `userId` field on documents in the
// `receipts` collection to find which userId has the most receipts. We limit
// the results to five and sort them in *descending order*. This gets us
// the list of the top five users with the most receipts.
db.receipts.aggregate([
  {
    $group: {
      _id: "$userId",
      count: { $sum: 1 }
    }
  }, {
    $sort: {
      count: -1
    }
  }, {
    $limit: 5
  }
])

Aggregate documents to get relevant analytic information.

// Aggregation is a pipeline. First, we `$match`, then we send those results
// to `$group`, then, we send those results to a different `$match` clause.

// In this example, we can aggregate our receipts to find documents where
// there was an item on sale, then group those receipts by the user.
// 1) `$match` items on sale that have a `userId` (every receipt should have one, but just being safe).
// 2) `$group` those receipts with sales by the `userId`.
// 3) `$match` those receipts grouped by `userId` and only return the results that have more than one match.

// This would get us documents for users who bought more than one item on sale.
// We could make this more interesting by adding something like 'purchaseDate' 
// to the first $match so that we can find users who bought more than one item on sale
// during a certain period.
db.receipts.aggregate(
  [
    {
      $match: {
        containsItemOnSale: true,
        userId: {
          $exists: true
        }
      }
    }, {
      $group: {
        _id: {
          userId: "$userId"
        },
        count: {
          $sum: 1
        }
      }
    }, {
      $match: {
        count: {$gt: 1}
      }
    }
  ]
)