Retrieving a document by ID in Elasticsearch - Mini Project: Build & Apply

Retrieving a document by ID

📖 Scenario: You are working with an Elasticsearch database that stores information about books in a library. Each book is stored as a document with a unique ID.You want to learn how to retrieve a specific book's details by using its document ID.

🎯 Goal: Build a simple Elasticsearch query to retrieve a document by its ID from the library index.

📋 What You'll Learn

Create a variable with the document ID to search for

Create a variable with the index name library

Write the Elasticsearch query to get the document by ID

Print the retrieved document's source content

💡 Why This Matters

🌍 Real World

Retrieving documents by ID is a common task in search engines and databases to quickly access specific records.

💼 Career

Knowing how to query Elasticsearch by document ID is important for backend developers, data engineers, and search specialists working with Elasticsearch.

1

Set up the document ID and index name

Create a variable called doc_id and set it to "book_123". Also create a variable called index_name and set it to "library".

Elasticsearch

  # Your code here

Hint

Use simple assignment to create the variables doc_id and index_name.

2

Import Elasticsearch client and create connection

Import Elasticsearch from elasticsearch and create a client instance called es with default settings.

Elasticsearch

doc_id = "book_123"
index_name = "library"
# Your code here

Hint

Use from elasticsearch import Elasticsearch and then es = Elasticsearch() to connect.

3

Retrieve the document by ID

Use the es.get() method with index=index_name and id=doc_id to get the document. Store the result in a variable called result.

Elasticsearch

from elasticsearch import Elasticsearch

doc_id = "book_123"
index_name = "library"
es = Elasticsearch()
# Your code here

Hint

Call es.get() with the correct index and id arguments.

4

Print the retrieved document source

Print the _source field from the result variable to display the document content.

Elasticsearch

from elasticsearch import Elasticsearch

doc_id = "book_123"
index_name = "library"
es = Elasticsearch()

result = es.get(index=index_name, id=doc_id)
# Your code here

Hint

Use print(result['_source']) to show the document details.

Practice

1. What is the fastest way to retrieve a single document in Elasticsearch?

easy

A. Retrieve the document by its ID using the GET API

B. Search all documents with a match_all query

C. Use a scroll API to iterate over documents

D. Run an aggregation query on the index

Solution

1. Step 1: Understand retrieval methods

   Retrieving by ID directly targets one document, making it the fastest method.

2. Step 2: Compare with other methods

   Search queries and scroll APIs scan multiple documents, which is slower than direct ID retrieval.

3. Final Answer:

   Retrieve the document by its ID using the GET API -> Option A

4. Quick Check:

   Fastest retrieval = GET by ID [OK]

Hint: Use GET API with document ID for fastest retrieval [OK]

Common Mistakes:

• Thinking search queries are faster than direct ID retrieval
• Using scroll API for single document retrieval
• Confusing aggregation with document retrieval

2. Which of the following is the correct syntax to retrieve a document by ID in Elasticsearch?

easy

A. GET /index/_doc/123

B. GET /index/_search {"query": {"term": {"_id": "123"}}}

C. POST /index/_doc/123/_get

D. GET /index/_search/123

Solution

1. Step 1: Recall the GET by ID syntax

   The correct syntax uses GET with the index and document ID: GET /index/_doc/123

2. Step 2: Identify incorrect options

   GET /index/_search {"query": {"term": {"_id": "123"}}} uses a search query, not direct GET by ID. POST /index/_doc/123/_get uses POST and an invalid path. GET /index/_search/123 has an invalid URL structure.

3. Final Answer:

   GET /index/_doc/123 -> Option A

4. Quick Check:

   Correct syntax = GET /index/_doc/ID [OK]

Hint: Use GET /index/_doc/{id} to retrieve document by ID [OK]

Common Mistakes:

• Using search API instead of GET by ID
• Wrong HTTP method like POST
• Incorrect URL path structure

3. Given the following request, what will Elasticsearch return if the document with ID 'abc123' exists?

GET /products/_doc/abc123

medium

A. HTTP 404 Not Found

B. {"_index":"products","_id":"abc123","found":false}

C. {"_index":"products","_id":"abc123","found":true,"_source":{"name":"Laptop","price":1200}}

D. {"error":"Document not found"}

Solution

1. Step 1: Understand the GET by ID response

   If the document exists, Elasticsearch returns found: true with the document source.

2. Step 2: Match the response format

   {"_index":"products","_id":"abc123","found":true,"_source":{"name":"Laptop","price":1200}} shows found: true and the document data, which matches expected output.

3. Final Answer:

   {"_index":"products","_id":"abc123","found":true,"_source":{"name":"Laptop","price":1200}} -> Option C

4. Quick Check:

   Document exists = found: true with _source [OK]

Hint: Found true means document exists with _source data [OK]

Common Mistakes:

• Expecting HTTP 404 instead of found: false
• Confusing error message with standard response
• Ignoring the _source field in response

4. You run this request to get a document by ID but get an error:

GET /store/_doc

What is the problem and how to fix it?

medium

A. The _doc endpoint does not support GET requests

B. Wrong HTTP method; use POST instead of GET

C. Index name is incorrect; change 'store' to '_store'

D. Missing document ID in URL; add the ID after _doc

Solution

1. Step 1: Identify the request format

   The GET by ID request requires the document ID after _doc in the URL.

2. Step 2: Fix the URL

   Add the document ID after _doc, e.g., GET /store/_doc/123

3. Final Answer:

   Missing document ID in URL; add the ID after _doc -> Option D

4. Quick Check:

   GET by ID needs document ID in URL [OK]

Hint: Always include document ID after _doc in GET URL [OK]

Common Mistakes:

• Omitting document ID in GET request URL
• Using wrong HTTP method
• Misnaming the index with underscore

5. You want to retrieve a document by ID but also handle the case when the document is missing without causing an error. Which approach is best?

hard

A. Use a search query with term _id and expect zero hits if missing

B. Use GET /index/_doc/id and check if 'found' is true in the response

C. Use GET /index/_doc/id and catch HTTP 404 error to handle missing

D. Use scroll API to find the document and handle empty results

Solution

1. Step 1: Understand GET by ID response behavior

   GET by ID returns a JSON with 'found': false if the document is missing, no error thrown.

2. Step 2: Compare with other methods

   Search queries return hits but are slower; catching HTTP 404 is unnecessary; scroll API is for multiple docs.

3. Final Answer:

   Use GET /index/_doc/id and check if 'found' is true in the response -> Option B

4. Quick Check:

   Check 'found' field to detect missing document [OK]

Hint: Check 'found' in GET response to detect missing docs safely [OK]

Common Mistakes:

• Expecting HTTP 404 error for missing documents
• Using search queries instead of GET by ID
• Using scroll API for single document retrieval