DynamoDB Dynomite Model Typecasting

By default, Dynomite uses aws-sdk-dynamodb to typecast by inferring the type from the value itself. Dynomite can also do some additional typecasting on top of the built-in aws-sdk-dynamodb typecasting.

The aws-sdk-dynamodb typecasting is based on inference of the value itself. For example, if you save a Ruby float value, it’s typecasted by aws-sdk-dynamodb to a DynamoDB number. If you save a Ruby string value, it’s saved as a DynamoDB string. See: github aws-sdk-dynamodb/attribute_value.rb. It’s all handled transparently for you. Whatever data-type value that’s stored is returned later.

Additional Typecasting

Dynomite performs additional typecasting for:

  • boolean: Dynomite typecasts type: :boolean for these values. These values [false, 'false', 'FALSE', 0, '0', 'f', 'F', 'off', 'OFF'] are falsey. All other values are truthy. It also provides an extra boolean ? method when you set the type: :boolean. IE: published?. This method does run a !! to ensure it returns a Boolean.
  • infer: Dynomite does not perform typecasting itself. Instead, it uses aws-sdk-dynamodb to handle the typecasting via inference of the value itself. This allows you to save Array, Set, and Map “types”. This is the default_field_type when you do not specific a type in the model field declartion.
  • integer: Dynomite typecasts type: :integer with a .to_i. Internally, it’s still stored as a native DynamoDB number. However, seeing Ruby float numbers when debugging can be annoying. For example, you’ll see 0.1e2 vs 10 in a jets console. Hence, the value is typecasted to a Ruby Integer. Integer typecasting can also be performed for comparison operations like Post.where("ttl.lt": Time.now) for it to work correctly.
  • string: Dynomite typecasts type: :string with a .to_s. This forces the value to be a String.
  • time: Since DynamoDB has no native time type, Dynomite stores a Time object as an ISO 8601 date string, IE: 2023-08-20T21:58:12Z per AWS Docs. Dynomite typecasts DateTime to a Time so everything handled as Time internally.

Typecasting Default

The default type casting behavior is to infer based on the value itself. You can change this with:

config/initializers/dynomite.rb

Dynomite.configure do |config|
  config.default_field_type = :infer # :string
end

Attribute Definitions

DynamoDB tables do have some “schema” information. This is in attribute_definitions. I believe DynamoDB uses this for optimizations because when you define a index for a field, the attribute definitions must also be provided and a type must be specified. AWS DynamoDB then enforces the type on the field. Because of this, Dynamodb does a final typecast on the value to match what the attribute definition type is, Number, String, Boolean before persisting the data. Otherwise, the value will not be allowed to be saved.

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