Adjacency list pattern in DynamoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to specify the parent ID attribute in the DynamoDB item.

DynamoDB

item = {"id": "123", "[1]": "456"}

Drag options to blanks, or click blank then click option'

AparentId

BchildId

CnodeId

DrootId

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'childId' instead of 'parentId' confuses the direction of the relationship.

Using generic names like 'nodeId' does not clarify the parent-child link.

In the adjacency list pattern, the parent ID is stored in an attribute typically named parentId.

2fill in blank

medium

Complete the code to query all items with a specific parent ID using DynamoDB's Query operation.

DynamoDB

response = table.query(KeyConditionExpression=Key('parentId').[1]('456'))

Drag options to blanks, or click blank then click option'

Abegins_with

Bcontains

Ceq

Dbetween

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'begins_with' when you want an exact match.

Using 'contains' which is for non-key attributes.

The eq method is used to find items where the key equals a specific value.

3fill in blank

hard

Fix the error in the code to correctly insert a child node with a parent reference.

DynamoDB

table.put_item(Item={"id": "789", "[1]": "123"})

Drag options to blanks, or click blank then click option'

AchildId

BparentId

Cparent_key

DrootId

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'childId' which is not the standard attribute for parent reference.

Using 'parent_key' which is not recognized by the schema.

The attribute must be named parentId to correctly represent the parent in the adjacency list.

4fill in blank

hard

Fill both blanks to create a DynamoDB query that retrieves all children of a given parent ID.

DynamoDB

response = table.query(KeyConditionExpression=Key('[1]').[2]('001'))

Drag options to blanks, or click blank then click option'

AparentId

Beq

Cbegins_with

DchildId

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'childId' instead of 'parentId' in the query.

Using 'begins_with' when an exact match is needed.

The query must use the parentId attribute with the eq method to find exact matches.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps child IDs to their parent IDs from a list of items.

DynamoDB

child_to_parent = {item['[1]']: item['[2]'] for item in items if item['[3]'] is not None}

Drag options to blanks, or click blank then click option'

Aid

BparentId

DchildId

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'childId' as the key which is not the standard attribute for the child's ID.

Not filtering out items where parentId is None.

The dictionary maps each child's id to its parentId, filtering out items without a parent.

Practice

1. What does the adjacency list pattern store in each item to represent relationships in DynamoDB?

easy

A. The parent item's identifier

B. The list of all child items

C. The full path from root to the item

D. The depth level of the item in the hierarchy

Solution

1. Step 1: Understand adjacency list pattern basics

   The adjacency list pattern stores a reference to the parent item for each node to represent hierarchy.

2. Step 2: Identify what is stored per item

   Each item contains the parent's ID, not the children or full path or depth.

3. Final Answer:

   The parent item's identifier -> Option A

4. Quick Check:

   Adjacency list = parent ID stored [OK]

Hint: Remember: adjacency list stores parent ID only [OK]

Common Mistakes:

• Thinking it stores all children in each item
• Confusing with storing full path or depth
• Assuming it stores unrelated metadata

2. Which of the following is the correct way to represent a parent-child relationship in a DynamoDB item using the adjacency list pattern?

easy

A. { "ItemId": "123", "Children": ["456", "789"] }

B. { "ItemId": "123", "ParentId": "456" }

C. { "ItemId": "123", "Path": "/root/456/123" }

D. { "ItemId": "123", "Depth": 2 }

Solution

1. Step 1: Identify adjacency list item structure

   Adjacency list stores the parent ID inside each item to link it to its parent.

2. Step 2: Match correct JSON structure

   { "ItemId": "123", "ParentId": "456" } shows an item with its own ID and a ParentId field, which is correct.

3. Final Answer:

   { "ItemId": "123", "ParentId": "456" } -> Option B

4. Quick Check:

   ParentId field present = adjacency list format [OK]

Hint: Look for ParentId key in item JSON [OK]

Common Mistakes:

• Using Children array instead of ParentId
• Including full path or depth instead of parent reference
• Confusing item ID with parent ID

3. Given the following DynamoDB items using adjacency list pattern:

{ "ItemId": "1", "ParentId": null }

{ "ItemId": "2", "ParentId": "1" }

{ "ItemId": "3", "ParentId": "1" }

{ "ItemId": "4", "ParentId": "2" }

If you query for items with ParentId = '1', which items will you get?

medium

A. [{ "ItemId": "2", "ParentId": "1" }, { "ItemId": "3", "ParentId": "1" }]

B. [{ "ItemId": "1", "ParentId": null }]

C. [{ "ItemId": "4", "ParentId": "2" }]

D. [{ "ItemId": "2", "ParentId": "1" }, { "ItemId": "3", "ParentId": "1" }, { "ItemId": "4", "ParentId": "2" }]

Solution

1. Step 1: Understand query condition

   The query filters items where ParentId equals '1'.

2. Step 2: Identify matching items

   Items with ParentId '1' are ItemId '2' and '3'. Item '4' has ParentId '2', so it does not match.

3. Final Answer:

   [{ "ItemId": "2", "ParentId": "1" }, { "ItemId": "3", "ParentId": "1" }] -> Option A

4. Quick Check:

   Filter ParentId='1' returns items 2 and 3 [OK]

Hint: Filter by ParentId to get direct children only [OK]

Common Mistakes:

• Including items with different ParentId values
• Confusing root item with children
• Assuming query returns grandchildren

4. You wrote this DynamoDB query to get children of item '5':

FilterExpression: "ParentId = :pid", ExpressionAttributeValues: { ":pid": "5" }

But it returns no results, even though you know children exist. What is the likely error?

medium

A. The ExpressionAttributeValues syntax is incorrect

B. You should use KeyConditionExpression instead of FilterExpression

C. You need to query with ItemId = '5' instead

D. ParentId attribute is missing or named differently in items

Solution

1. Step 1: Check attribute naming

   If ParentId attribute is missing or named differently, filter finds no matches.

2. Step 2: Verify attribute presence in items

   Ensure all child items have ParentId attribute exactly named 'ParentId'. If not, query returns empty.

3. Final Answer:

   ParentId attribute is missing or named differently in items -> Option D

4. Quick Check:

   Attribute name mismatch causes no results [OK]

Hint: Check attribute names match exactly in query and items [OK]

Common Mistakes:

• Confusing FilterExpression with KeyConditionExpression
• Incorrect syntax in ExpressionAttributeValues
• Querying by ItemId instead of ParentId

5. You want to retrieve the full hierarchy path from a child item up to the root using the adjacency list pattern in DynamoDB. Which approach is best?

hard

A. Use a DynamoDB join operation to combine parent and child items

B. Store the full path string in each item and query it directly

C. Iteratively query each item's ParentId until reaching the root

D. Use a single query filtering all items with ParentId equal to the child's ID

Solution

1. Step 1: Understand adjacency list limitations

   Adjacency list stores only parent references, so full path is not stored.

2. Step 2: Retrieve full path by iterative queries

   To get full path, query each item's parent repeatedly until root (ParentId null) is reached.

3. Step 3: Evaluate other options

   Store the full path string in each item and query it directly is a different pattern (materialized path). Use a single query filtering all items with ParentId equal to the child's ID gets children, not parents. Use a DynamoDB join operation to combine parent and child items is invalid as DynamoDB has no join.

4. Final Answer:

   Iteratively query each item's ParentId until reaching the root -> Option C

5. Quick Check:

   Adjacency list needs iterative parent queries for full path [OK]

Hint: Follow parent links step-by-step to build full path [OK]

Common Mistakes:

• Expecting DynamoDB to support joins
• Assuming full path is stored in adjacency list
• Trying to get full path with single query filtering children