Boto3 KeyConditionExpression works and get_item does not

I am new to DynamoDB and finding my way around DocumentDBs versus Relational DBs.

My code is running via AWS Lambda functions with a Python runtime. It works.

def get_item_by_id(table_name: str, partition_key: str, partition_value: str) -> List[dict]:
    """
    Retrieve items from DynamoDB table based on the partition key and its value.
    Args:
        table_name: The name of the DynamoDB table.
        partition_key: The partition key of the table.
        partition_value: The value of the partition key to search for.
    Returns:
        A list of items matching the partition key and value.
    Raises:
        boto3.exceptions.ResourceNotFoundException: If the DynamoDB table does not exist.
    """
    dynamodb = resource('dynamodb')
    table = dynamodb.Table(table_name)
    
    response = table.query(
        KeyConditionExpression=Key(partition_key).eq(partition_value)
    )
    
    items = response['Items']
    
    return items

However, it took me a couple of hours to find from boto3.dynamodb.conditions import Key in the official documentation.

Instead, the official documentation "code examples" refers to a function called get_item():

response = table.get_item(
    Key={
        'username': 'janedoe',
        'last_name': 'Doe'
    }
)
item = response['Item']
print(item)

Below was my version

def get_item_by_id(table_name: str, item_id: str) -> Optional[Dict[str, Any]]:
    """
    Retrieve an item from DynamoDB table based on the item ID.
    Args:
        table_name: The name of the DynamoDB table.
        item_id: The ID of the item to retrieve.
    Returns:
        The retrieved item as a dictionary, or None if the item is not found.
    Raises:
        boto3.exceptions.ResourceNotFoundException: If the DynamoDB table does not exist.
    """
    dynamodb = boto3.resource('dynamodb')
    table = dynamodb.Table(table_name)
    response = table.get_item(Key={'ID': item_id})
    item = response.get('Item')
    return item

This was never able to return a successful response, this was the error:

> "errorMessage": "An error occurred (ValidationException) when calling the GetItem operation: The provided key element does not match the schema"

Can I ask:

1. Why does KeyConditionExpression work with identical table names, attribute name and value?
2. Could I have ever got get_item() working?

Thank you.

You are trying to compare 2 different APIs.

GetItem

This method targets a single item in a table, and you must provide the entire primary key which is partition and sort key.

You are seeing the exception as you do not provide the sort key name or value.

Query

This method uses a KeyConditionExpression and targets multiple items which share the same partition key. This method only requires you to provide the partition key, and you can decide to exclude the sort key, or use a conditional function on the sort key such as between.

Documentation

I would strongly advise that you take a couple of hours and read the DynamoDB documentation thoroughly, it'll make a great difference to you understanding instead of wasting hours writing code which doesn't work.

From what I've seen:

KeyConditionExpression=Key(partition_key).eq(partition_value)

is more relaxed on data types while on the other side

response = table.get_item(Key={'ID': item_id})

is strict on the data scheme you setup with dynamodb. item_id: str looks suspect if the ID is a numeric value instead of a string. See the type mappings here