Write to DynamoDB

If you've selected DynamoDB as your database of choice, this chapter will look at how to store our comments in DynamoDB.

If this is your first time using DynamoDB, it's quite different from relational databases like PostgreSQL or MySQL. It's best used with a pattern called Single Table Design.

Single Table Design

Single Table Design can be a bit of work to learn but create sst ships with an excellent library called ElectroDB. It provides a simplified way of using the pattern.

info

ElectroDB helps us get started and use Single Table Design in DynamoDB.

While you should eventually dig deeper and learn the underlying pattern, ElectroDB helps you quickly get started and scales well to the most advanced use cases.

Define Schema

In ElectroDB each concept in your data model is called an Entity and you can specify right in your application code.

Let's start by creating a new Entity for comments in services/core/article.ts.

services/core/article.ts

const CommentEntity = new Entity(
  {
    model: {
      version: "1",
      entity: "Comment",
      service: "myapp",
    },
    attributes: {
      commentID: {
        type: "string",
        required: true,
        readOnly: true,
      },
      articleID: {
        type: "string",
        required: true,
        readOnly: true,
      },
      text: {
        type: "string",
        required: true,
      },
    },
    indexes: {
      primary: {
        pk: {
          field: "pk",
          composite: ["commentID"],
        },
        sk: {
          field: "sk",
          composite: [],
        },
      },
      byArticle: {
        index: "gsi1",
        pk: {
          field: "gsi1pk",
          composite: ["articleID"],
        },
        sk: {
          field: "gsi1sk",
          composite: ["commentID"],
        },
      },
    },
  },
  Dynamo.Configuration
);

We are doing a couple of things here:

• We define a new CommentEntity with the fields articleID, commentID, and text. Where articleID and commentID are marked as required and readOnly. This means that it cannot be changed after creation.

  Additionally, it uses the indexes defined in stacks/Database.ts to implement some useful functionality. The primary index, which is required, will allow querying this object by the commentID to retreive a single comment given its ID.

• The byArticle is a secondary index that allows fetching a list of comments given the articleID. ElectroDB handles implementing these indexes through Single Table Design and you can learn more about this here.

Implementation

Now let's implement the addComment and comments functions that we created back in the Scaffold Business Logic chapter.

Replace the two placeholder functions in services/core/article.ts with:

services/core/article.ts

export async function addComment(articleID: string, text: string) {
  return CommentEntity.create({
    articleID,
    commentID: ulid(),
    text,
  }).go();
}

export async function comments(articleID: string) {
  return CommentEntity.query
    .byArticle({
      articleID,
    })
    .go();
}

Now with our business logic implemented, we are ready to hook up our API.