Read Data from MongoDB Atlas - Functions

Find a Single Document (findOne())

You can find a single document using the collection.findOne() method.

The following function snippet finds a single document from the items collection that has a quantity greater than or equal to 25:

const query = { "quantity": { "$gte": 25 } };
const projection = {
 "title": 1,
 "quantity": 1,
}
return itemsCollection.findOne(query, projection)
  .then(result => {
    if(result) {
      console.log(`Successfully found document: ${result}.`);
    } else {
      console.log("No document matches the provided query.");
    }
    return result;
  })
  .catch(err => console.error(`Failed to find document: ${err}`));

Find One or More Documents (find())

You can find multiple documents using the collection.find() method.

The following function snippet finds all documents in the items collection that have at least one review and returns them sorted by name with the _id field omitted:

const query = { "reviews.0": { "$exists": true } };
const projection = { "_id": 0 };
return itemsCollection.find(query, projection)
  .sort({ name: 1 })
  .toArray()
  .then(items => {
    console.log(`Successfully found ${items.length} documents.`)
    items.forEach(console.log)
    return items
  })
  .catch(err => console.error(`Failed to find documents: ${err}`))

Count Documents in the Collection (count())

You can count documents in a collection using the collection.count() method. You can specify a query to control which documents to count. If you don't specify a query, the method counts all documents in the collection.

The following function snippet counts the number of documents in the items collection that have at least one review:

return itemsCollection.count({ "reviews.0": { "$exists": true } })
  .then(numDocs => console.log(`${numDocs} items have a review.`))
  .catch(err => console.error("Failed to count documents: ", err))

Find by Document ID

You can query a collection to find a document that has a specified ID. MongoDB automatically stores each document's ID as an ObjectId value in the document's _id field.

{ "_id": <ObjectId> }

{ "_id": BSON.ObjectId("5ad84b81b8b998278f773c1b") }

Find by Date

You can query a collection to find documents that have a field with a specific date value, or query for a documents within a range of dates.

{ "<Date Field Name>": <Date | Expression> }

{ "createdAt": new Date("2019-01-23T05:00:00.000Z") }

{
  "createdAt": {
    "$gte": new Date("2019-01-01T00:00:00.000Z"),
    "$lt": new Date("2020-01-01T00:00:00.000Z"),
  }
}

Match a Root-Level Field

You can query a collection based on the value of a root-level field in each document. You can specify either a specific value or a nested expression that MongoDB evaluates for each document.

For more information, see the Query Documents tutorial in the MongoDB Server Manual.

{ "<Field Name>": <Value | Expression> }

{ "name": "Basketball" }

Match Multiple Fields

You can specify multiple query conditions in a single query document. Each root-level field of a query document maps to a field in the collection. MongoDB only returns documents that fulfill all query conditions.

For more information, see the Query on Embedded/Nested Documents tutorial in the MongoDB Server Manual.

{
  "<Field Name 1>": <Value | Expression>,
  "<Field Name 2>": <Value | Expression>
}

{
  "name": "Basketball",
  "quantity": { "$gt": 0 }
}

Match an Embedded Document Field

You can query a collection based on the value of embedded document fields. To specify an embedded document field, use multiple nested query expressions or standard document dot notation.

For more information, see the Query on Embedded/Nested Documents tutorial in the MongoDB Server Manual.

{ "<Field Name>": { "<Nested Field Name>": <Value | Expression> } }

{ "<Field Name>.<Nested Field Name>": <Value | Expression> }

{
  "reviews.0.username": "JoeMango"
}

Match an Array of Values

You can query a collection based on all the elements contained in an array field.

If you query an array field for a specific array of values, MongoDB returns documents where the array field exactly matches the specified array of values. If you want MongoDB to return documents where the array field contains all elements in the specified array of values, use the $all operator.

For more information, see the Query an Array tutorial in the MongoDB Server Manual.

{ "<Array Field Name>": [<Value>, ...] }

{
  "reviews": [{ username: "JoeMango", comment: "This rocks!" }]
}

{
  "reviews": {
    "$all": [{ username: "JoeMango", comment: "This rocks!" }]
  }
}

Match an Array Element

You can query a collection based on the value of one or more elements in an array field.

If you query an array field with a query expression that has multiple conditions, MongoDB returns documents where any combination of the array's elements satisfy the expression. If you want MongoDB to return documents where a single array element satisfies all of the expression conditions, use the $elemMatch operator.

For more information, see the Query an Array tutorial in the MongoDB Server Manual.

{ "<Array Field Name>": <Value | Expression> }

{
  "reviews": {
    "username": "JoeMango",
    "comment": "This is a great product!"
  }
}

{
  "reviews": {
    "$elemMatch": {
      "username": "JoeMango",
      "comment": "This is a great product!"
    }
  }
}

Compare Values

You can use a comparison operator to compare the value of a document field to another value.

{ "<Field Name>": { "<Comparison Operator>": <Comparison Value> } }

The following comparison operators are available:

{
  "quantity": { "$gt": 0, "$lte": 10 }
}

Evaluate a Logical Expression

You can use a logical operator to evaluate multiple expressions for a single field.

{
  "<Field Name>": {
    "<Logical Operator>": [<Expression>, ...]
  }
}

The following logical operators are available:

{
  "$or": [
    { "quantity": { "$gt": 0 } },
    { "reviews": { "$size": { "$lte": 5 } } }
  ]
}

Evaluate a Regular Expression

You can use the $regex query operator to return documents with fields that match a regular expression. To avoid ambiguity with the $regex EJSON type, you must use a atlas-BSON.BSONRegExp object.

{
  "<Field Name>": {
    "$regex": BSON.BSONRegExp(<RegEx String>, <RegEx Options>)
  }
}

{
  "name": { "$regex": BSON.BSONRegExp(".+ball", "i") }
}