# MongoDB

> An overview of MongoDB from my perspective and experience.

Source: https://curiouslychase.com/engineering/mongodb/

# Common Query Patterns

This is a collection built up over time of queries I use and find helpful.

- [update or add a property to subdocuments of all found documents (MongoDB)](https://curiouslychase.com/engineering/update-or-add-a-property-to-subdocuments-of-all-found-documents-mongodb/)

# Document Arrays

Find any documents where key `smartEvents` exist and has an array length that's greater than 0:

```
{smartEvents: { $exists: true, $not: { $size: 0 } } }
```

Find any subdocuments of `governs` and an array key for `groups` where `groups` contains an `ObjectID` of `622b8fe0968213000946ee1e`

```
{"governs.groups":{ $in: [ObjectId('622b8fe0968213000946ee1e')]}}
```

## $elemMatch
`$elemMatch` is a useful operator for when you want to search an array of subdocuments where a field contains a value. In this example, `responses` is an array with a document that has a field of `content`:

```
{responses: {$elemMatch: {"content": {$regex:"Maybe tolerances could"}}}}
```

Also note `$regex` in the query above. This operator lets you search for a regular expression (or a plain string) within a value. This is useful when you don't want to search the whole value of a property in a document.

# MongoDB Compass

I use Compass pretty heavily for querying data in MongoDB.