3 fields composite primary key (unique item) in Dynamodb

Question

I am trying to create a table to store invoice line items in DynamoDB. Let's say the item is defined by CompanyCode, InvoiceNumber and LineItemId, amount and other line item details.

A unique item is defined by the combination of the first 3 attributes. Any 2 of those attributes can be same for the different items. What should I select as the Hash Attribute and the Range Attribute?

Answer 1

Some Intro

For efficiency I would propose totally different design. With NoSQL databases (and DynamoDB is not different) we always need to consider the access patterns first. Also, if possible we should strive to fit all our data within same table and several indexes. From what we have from OP and his comments, these are the two access patterns:

1. For a company X, get complete invoice Y (including all items or range of items) [based on this comment ]
2. Get all invoices for company X [ based on this comment ]

We now wonder what is a good Primary Key? Translates to question what is a good Partition Key (PK) and what is a good Sort Key (SK) and which secondary indexes do we need to create and of what kind (local or global)? Some reminders:

• Primary Key can be on one column or composite
• Composite primary key consists of Partition Key and Sort Key
• Partition key is used as input to the hashing function that will determine partition of the items
• Sort key can also be composite, which allows us to model one-to-many relationships in DynamoDB as given in one of the comments links: https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/bp-sort-keys.html
• When creating query on the table or index, you always need to use '=' operator on the Partition Key
• When querying ranges on Sort Key you have option for KeyConditionExpression which provides you with set of operators for sorting and everything in between (one of them being function begins_with (a, substr) )
• You are also allowed to use FilterExpression if you need to further refine the Query results (filter on the projected attributes)
• Local Secondary Indexes (LSI) have same Partition Key but different Sort Key than your original table and give you different view of your data, organized according to an alternative Sort Key
• Global Secondary Indexes (GSI) have different Partition Key and different Sort Key than your original table and give you completely different view on data
• All items with the same partition key are stored together, and for composite Primary keys, are ordered by the sort key value. DynamoDB splits partitions by sort key if the collection size grows bigger than 10 GB.

Back To Modeling

It is obvious that we are dealing with multiple entities that need to be modeled and fit into the same table. To satisfy condition of Partition Key being unique on the table, CompanyCode comes as a natural Partition Key - so I would ensure that is unique. If not then you need to ask yourself how can you model the second access pattern?

Assuming we have established uniqueness on the CompanyCode let's simplify and say that it comes in the form of an e-mail (or could be domain or just a code, but I will use email for demonstration).

• Relationship between Company and Invoices is always 1:many.
• Relationship between Invoice and Items is always 1:many.

I propose design as in the image below:

• With PK being CompanyCode and SK being InvoiceNumber can store all attributes about that invoice for that company.
• Nothing prevents me to also add record where the SK is Customer which allows me to store all attributes about the company.
• With GSI1 , we will create reverse lookup where GSI1PK is my tables SK (InvoiceNumber) and my GSI1SK is my tables PK (CompanyCode).
• I am using same table to store line items with PK being LineItemId and SK being CompanyCode (still unique)
• For Item entity items my GSI1PK is still InvoiceNumber and my GSI1SK is LineItemId which is tables PK so its same as for Invoice entity items.

Now the access patterns supported with this:

• If I want to get invoice Y for company X and all the items (access pattern 1): Query the table where CompanyCode=X and use KeyConditionExpression with = operator on the Sort Key InvoiceNumber. If I want to get all the items tied to that invoice, I will project Items attribute using ProjectionExpression.
• By retrieving all the items with previous query for company X and invoice Y, I can now run BatchGetItem API call (using my unique composite key LineItemId+CompanyCode) on table to get all items belonging to that particular invoice of that particular customer. (this comes with some constraints of BatchGetItem API)
• To support access pattern 2, I will do a query with CompanyCode=X on PK and use KeyConditionExpression on the SK with begins_with (a, substr) function/operator to get only invoices for company X and not the metadata about that company. That will give me all invoices for given company/customer.
• Additionally, with above GSI1, for any given InvoiceNumber I can easily select all the line items that belong to that particular invoice. REMEMBER: The key values in a global secondary index do not need to be unique - so in my GSI1 I could have had easily invoice_1 -> (item_1, item_2) and then another invoice_1 -> (item_1,item_2) but the difference between two items in GSI would be in the SK (it would be associated with different CompanyCode (but for demonstration purposes I used invoice_1 and invoice_2).