Nested Objects in Elasticsearch

Nested objects in Elasticsearch allow us to index arrays of objects as a single field, which is useful when dealing with arrays of objects that need to be queried or filtered as a single entity. Here’s how nested objects work:

• Indexing Nested Objects: When we index a document with a nested object, Elasticsearch indexes each object in the array as a separate hidden document, but maintains the relationship between the objects.
• Querying Nested Objects: To query nested objects we need to use a nested query, which allows us to query the nested objects as if they were indexed as separate documents.
• Mapping for Nested Objects: To use nested objects we need to define a mapping for the field as type “nested“. This tells Elasticsearch to treat the field as a nested object.

Example: Using Nested Objects

Suppose we have an index for storing blog posts and each post can have multiple comments. We can use a nested object to represent the comments within each post.

PUT /blog-posts
{
  "mappings": {
    "properties": {
      "title": { "type": "text" },
      "content": { "type": "text" },
      "comments": {
        "type": "nested",
        "properties": {
          "username": { "type": "keyword" },
          "comment": { "type": "text" },
          "created_at": { "type": "date" }
        }
      }
    }
  }
}

Explanation:

• We define a comments field as a nested object within the blog-posts index.
• Each comment has properties like username, comment, and created_at.
• By using a nested object, we can query and aggregate comments independently of each other.

Nested objects and parent-child relationships in Elasticsearch are advanced features that enable us to model complex data structures and relationships within our indices. By understanding and using these features, we can improve the organization of our data and enhance the efficiency of our queries.

In this article, we will explore these concepts in detail Nested Objects, Parent-Child Relationships also Querying Nested Objects and Parent-Child Relationships in a beginner-friendly manner.