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Writing aggregations

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Writing aggregations

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NEST allows you to write your aggregations using

a strict fluent DSL
a verbatim object initializer syntax that maps verbatim to the Elasticsearch API
a more terse object initializer aggregation DSL

Three different ways, yikes that’s a lot to take in! Let’s go over them one at a time and explain when you might want to use each.

This is the json output for each example

{
  "aggs": {
    "name_of_child_agg": {
      "children": {
        "type": "commits"
      },
      "aggs": {
        "average_per_child": {
          "avg": {
            "field": "confidenceFactor"
          }
        },
        "max_per_child": {
          "max": {
            "field": "confidenceFactor"
          }
        },
        "min_per_child": {
          "min": {
            "field": "confidenceFactor"
          }
        }
      }
    }
  }
}

Fluent DSL

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The fluent lambda syntax is the most terse way to write aggregations. It benefits from types that are carried over to sub aggregations

Fluent DSL example

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s => s
.Aggregations(aggs => aggs
    .Children<CommitActivity>("name_of_child_agg", child => child
        .Aggregations(childAggs => childAggs
            .Average("average_per_child", avg => avg.Field(p => p.ConfidenceFactor))
            .Max("max_per_child", avg => avg.Field(p => p.ConfidenceFactor))
            .Min("min_per_child", avg => avg.Field(p => p.ConfidenceFactor))
        )
    )
)

Object Initializer syntax

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The object initializer syntax (OIS) is a one-to-one mapping with how aggregations have to be represented in the Elasticsearch API. While it has the benefit of being a one-to-one mapping, being dictionary based in C# means it can gow verbose rather quickly.

Here’s the same aggregations as expressed in the Fluent API above, with the dictionary-based object initializer syntax

Object Initializer syntax example

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new SearchRequest<Project>
{
    Aggregations = new AggregationDictionary
    {
        { "name_of_child_agg", new ChildrenAggregation("name_of_child_agg", typeof(CommitActivity))
            {
                Aggregations = new AggregationDictionary
                {
                    { "average_per_child", new AverageAggregation("average_per_child", "confidenceFactor") },
                    { "max_per_child", new MaxAggregation("max_per_child", "confidenceFactor") },
                    { "min_per_child", new MinAggregation("min_per_child", "confidenceFactor") },
                }
            }
        }
    }
}

This starts to get hard to read, wouldn’t you agree? There is a better way however…

Terse Object Initializer syntax

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The Object Initializer syntax can be shortened dramatically by using *Aggregation types directly, allowing you to forego the need to introduce intermediary dictionaries to represent the aggregation DSL. In using these, it is also possible to combine multiple aggregations using the bitwise && operator.

Compare the following example with the previous vanilla Object Initializer syntax

Object Initializer syntax example

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new SearchRequest<Project>
{
    Aggregations = new ChildrenAggregation("name_of_child_agg", typeof(CommitActivity))
    {
        Aggregations =
            new AverageAggregation("average_per_child", Field<CommitActivity>(p => p.ConfidenceFactor))
            && new MaxAggregation("max_per_child", Field<CommitActivity>(p => p.ConfidenceFactor))
            && new MinAggregation("min_per_child", Field<CommitActivity>(p => p.ConfidenceFactor))
    }
}

Now that’s much cleaner! Assigning an *Aggregation type directly to the Aggregation property on a search request works because there are implicit conversions within NEST to handle this for you.

Aggregating over a collection of aggregations

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An advanced scenario may involve an existing collection of aggregation functions that should be set as aggregations on the request. Using LINQ’s .Aggregate() method, each function can be applied to the aggregation descriptor (childAggs below) in turn, returning the descriptor after each function application.

Fluent DSL example

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var aggregations = new List<Func<AggregationContainerDescriptor<CommitActivity>, IAggregationContainer>> 
{
    a => a.Average("average_per_child", avg => avg.Field(p => p.ConfidenceFactor)),
    a => a.Max("max_per_child", avg => avg.Field(p => p.ConfidenceFactor)),
    a => a.Min("min_per_child", avg => avg.Field(p => p.ConfidenceFactor))
};

return s => s
    .Aggregations(aggs => aggs
        .Children<CommitActivity>("name_of_child_agg", child => child
            .Aggregations(childAggs =>
                aggregations.Aggregate(childAggs, (acc, agg) => { agg(acc); return acc; }) 
            )
        )
    );

	a list of aggregation functions to apply
	Using LINQ’s `Aggregate()` function to accumulate/apply all of the aggregation functions

Combining multiple AggregationDescriptor is also possible using the bitwise && operator

var aggregations = new AggregationContainerDescriptor<CommitActivity>()
        .Average("average_per_child", avg => avg.Field(p => p.ConfidenceFactor))
        .Max("max_per_child", avg => avg.Field(p => p.ConfidenceFactor))
        && new AggregationContainerDescriptor<CommitActivity>()
            .Min("min_per_child", avg => avg.Field(p => p.ConfidenceFactor));

return s => s
        .Aggregations(aggs => aggs
            .Children<CommitActivity>("name_of_child_agg", child => child
                .Aggregations(childAggs => aggregations)
            )
        );

Aggs vs. Aggregations

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The response exposes both .Aggregations and .Aggs properties for handling aggregations. Why two properties you ask? Well, the former is a dictionary of aggregation names to IAggregate types, a common interface for aggregation responses (termed Aggregates in NEST), and the latter is a convenience helper to get the right type of aggregation response out of the dictionary based on a key name.

This is better illustrated with an example. Let’s imagine we make the following request

s => s
.Aggregations(aggs => aggs
    .Children<CommitActivity>("name_of_child_agg", child => child
        .Aggregations(childAggs => childAggs
            .Average("average_per_child", avg => avg.Field(p => p.ConfidenceFactor))
            .Max("max_per_child", avg => avg.Field(p => p.ConfidenceFactor))
            .Min("min_per_child", avg => avg.Field(p => p.ConfidenceFactor))
        )
    )
)

Now, using .Aggs, we can easily get the Children aggregation response out and from that, the Average and Max sub aggregations.

Handling Responses

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response.IsValid.Should().BeTrue();

var childAggregation = response.Aggs.Children("name_of_child_agg");

var averagePerChild = childAggregation.Average("average_per_child");

averagePerChild.Should().NotBeNull(); 

var maxPerChild = childAggregation.Max("max_per_child");

maxPerChild.Should().NotBeNull();

	Do something with the average per child. Here we just assert it’s not null
	Do something with the max per child. Here we just assert it’s not null

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