DynamoDB Dynomite Querying Project Expression

Project Expression

You can use project or project_expression to “project” specific fields only. Both strings and splat args work.

Product.where(category: "Electronics").project(:id, :name, :category, :price)
Product.where(category: "Electronics").project("id, name, category, price")
Product.where(category: "Electronics").project("id, name", :category, :price) # mixing works too

Only the projected fields will be available. This is like using SQL select to grab only specific fields from the database.

product = Product.where(category: "Electronics").project(:id, :name, :category).first
product.name  # available
product.price # unavailable. it was not projected

This optimization can save DynamoDB from retrieving more data than is needed over the wire.

Start From or Exclusive Start Key

The start_from and exclusive_start_key are aliases of each other.

product = Product.first

Note, these are all aliases: start start_from start_at exclusive_start_key


The pluck is a quick way to project by only projecting the fields requested.

> Product.where(category: "Electronics").pluck(:id)
=> ["product-Ti4AYV8kwmW8iEib", "product-f41iTl98xPLwrVr0"]
> Product.where(category: "Electronics").pluck(:id, :name)
=> [["product-Ti4AYV8kwmW8iEib", "Laptop"], ["product-f41iTl98xPLwrVr0", "Smartphone"]]


You can use ids to quickly grab all the ids.

Product.ids                                 # without key_condition will perform a scan
Product.where(category: "Electronics").ids  # will perform query given category index

The ids will perform a scan unless provided a where clause that can use an index and key_condition.


The exists? method can check for existence.

Product.where(category: "Electronics").exists?

Exists does an optimization by using limit(1).first internally.