# Unwind

## Array deconstruction

The **$unwind** operator is used to deconstruct array fields.  How deconstruction works :question: For each array element a new document is produced having that element in the position of the field array. This means that applying the unwind operator on a single document's array field which contains 10 elements, produces 10 different documents having one of the array elements instead of the array field. It is possible to define if a new document will produced in case a document either doesn't contain the array field or it is empty.

{% tabs %}
{% tab title="Document" %}

```javascript
{
	"_id" : ObjectId("5e9afcd20f86e454c4518d8d"),
	"name" : "Margarett Lind",
	"age" : 59,
	"activities" : [
		"Golf",
		"Photography",
		"Hacking"
	]
}
```

{% endtab %}

{% tab title="$unwind" %}

```javascript
// $unwind on activities
db.trips.aggregate()
    .unwind("$activities")

// for each activity produces one document

/* 1 */
{
	"_id" : ObjectId("5e9afcd20f86e454c4518d8d"),
	"name" : "Margarett Lind",
	"age" : 59,
	"activities" : "Golf"
	
},

/* 2 */
{
	"_id" : ObjectId("5e9afcd20f86e454c4518d8d"),
	"name" : "Margarett Lind",
	"age" : 59,
	"activities" : "Photography"
},

/* 3 */
{
	"_id" : ObjectId("5e9afcd20f86e454c4518d8d"),
	"name" : "Margarett Lind",
	"age" : 59,
	"activities" : "Hacking"
}
```

{% endtab %}
{% endtabs %}

The following sample finds the distinct activities grouped by age for `Traveler` documents.

{% tabs %}
{% tab title="C#" %}
{% code title="Unwind.cs" %}

```csharp
var travelersQueryableCollection = tripsDatabase
    .GetCollection<Traveler>(Constants.TravelersCollection)
    .AsQueryable();

var linqQuery = travelersQueryableCollection
    .SelectMany(t => t.Activities, (t, a) => new
    {
        age = t.Age,
        activity = a
    })
    .GroupBy(q => q.age)
    .Select(g => new { age = g.Key, activities = 
        g.Select(a => a.activity).Distinct() })
    .OrderBy(r => r.age);

var linqQueryResults = await linqQuery.ToListAsync();
```

{% endcode %}
{% endtab %}

{% tab title="Shell" %}

```javascript
db.travelers.aggregate([
   {
      "$unwind":"$activities"
   },
   {
      "$project":{
         "age":"$age",
         "activity":"$activities",
         "_id":0
      }
   },
   {
      "$group":{
         "_id":"$age",
         "__agg0":{
            "$addToSet":"$activity"
         }
      }
   },
   {
      "$project":{
         "age":"$_id",
         "activities":"$__agg0",
         "_id":0
      }
   },
   {
      "$sort":{
         "age":1
      }
   }
])
```

{% endtab %}

{% tab title="Result" %}

```javascript
// sample results

/* 1 */
{
	"age" : 22,
	"activities" : [
		"Photography",
		"Road Touring"
	]
},

/* 2 */
{
	"age" : 25,
	"activities" : [
		"Photography",
		"Blogging",
		"Baking",
		"Collecting",
		"Hacking"
	]
},

/* 3 */
{
	"age" : 30,
	"activities" : [
		"Road Touring",
		"Climbing"
	]
},

/* 4 */
{
	"age" : 34,
	"activities" : [
		"Photography"
	]
}
```

{% endtab %}

{% tab title="Traveler" %}

```csharp
public class Traveler
{
    [BsonId]
    public ObjectId Id { get; set; }
    public string Name { get; set; }
    public int Age { get; set; }
    public List<string> Activities { get; set; }
    public List<VisitedCountry> VisitedCountries { get; set; }
}
```

{% endtab %}
{% endtabs %}

{% hint style="info" %}

### LINQ query explanation

* `SelectMany` on *Activities* creates the *$unwind* stage
* `new { }` creates the *$project* stage
* `GroupBy` groups documents by age field
* `Select`creates another *$project* stage where `Distinct` ensures activities will not contain duplicate values per group - it does that by using **$addToSet** operator in the *$group* stage rather than a **$push**
  {% endhint %}


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://chsakell.gitbook.io/mongodb-csharp-docs/aggregation/unwind-stage.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.