Merge pull request #374 from Dynamoid/add-project-method

Add Criteria::Chain#project method

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 ## Features
 
+* Feature: [#374](https://github.com/Dynamoid/dynamoid/pull/374) Add `#project` query method to load only specified fields
+
 ## Improvements
 
 * Improvement: [#359](https://github.com/Dynamoid/dynamoid/pull/359) Add support of `NULL` and `NOT_NULL` operators

README.md

@@ -674,6 +674,32 @@ operators check attribute presence in a document, not value.
 So if attribute `postcode`'s value is `NULL`, `NULL` operator will return false
 because attribute exists even if has `NULL` value.
 
+#### Selecting some specific fields only
+
+It could be done with `project` method:
+
+```ruby
+class User
+  include Dynamoid::Document
+  field :name
+end
+
+User.create(name: 'Alex')
+user = User.project(:name).first
+
+user.id         # => nil
+user.name       # => 'Alex'
+user.created_at # => nil
+```
+
+Returned models with have filled specified fields only.
+
+Several fields could be specified:
+
+```ruby
+user = User.project(:name, :created_at)
+```
+
 ### Consistent Reads
 
 Querying supports consistent reading. By default, DynamoDB reads are eventually consistent: if you do a write and then a read immediately afterwards, the results of the previous write may not be reflected. If you need to do a consistent read (that is, you need to read the results of a write immediately) you can do so, but keep in mind that consistent reads are twice as expensive as regular reads for DynamoDB.

lib/dynamoid/adapter_plugin/aws_sdk_v3/query.rb

@@ -11,6 +11,7 @@ class Query
         OPTIONS_KEYS = %i[
           limit hash_key hash_value range_key consistent_read scan_index_forward
           select index_name batch_size exclusive_start_key record_limit scan_limit
+          project
         ].freeze
 
         attr_reader :client, :table, :options, :conditions
@@ -63,10 +64,11 @@ def build_request
           batch_size = options[:batch_size]
           limit = [record_limit, scan_limit, batch_size].compact.min
 
-          request[:limit]          = limit if limit
-          request[:table_name]     = table.name
-          request[:key_conditions] = key_conditions
-          request[:query_filter]   = query_filter
+          request[:limit]             = limit if limit
+          request[:table_name]        = table.name
+          request[:key_conditions]    = key_conditions
+          request[:query_filter]      = query_filter
+          request[:attributes_to_get] = attributes_to_get
 
           request
         end
@@ -117,6 +119,11 @@ def query_filter
             result
           end
         end
+
+        def attributes_to_get
+          return if options[:project].nil?
+          options[:project].map(&:to_s)
+        end
       end
     end
   end

lib/dynamoid/adapter_plugin/aws_sdk_v3/scan.rb

@@ -54,9 +54,10 @@ def build_request
           batch_size = options[:batch_size]
           limit = [record_limit, scan_limit, batch_size].compact.min
 
-          request[:limit]       = limit if limit
-          request[:table_name]  = table.name
-          request[:scan_filter] = scan_filter
+          request[:limit]             = limit if limit
+          request[:table_name]        = table.name
+          request[:scan_filter]       = scan_filter
+          request[:attributes_to_get] = attributes_to_get
 
           request
         end
@@ -81,6 +82,11 @@ def scan_filter
             result
           end
         end
+
+        def attributes_to_get
+          return if options[:project].nil?
+          options[:project].map(&:to_s)
+        end
       end
     end
   end

lib/dynamoid/criteria.rb

@@ -8,7 +8,7 @@ module Criteria
     extend ActiveSupport::Concern
 
     module ClassMethods
-      %i[where all first last each record_limit scan_limit batch start scan_index_forward find_by_pages].each do |meth|
+      %i[where all first last each record_limit scan_limit batch start scan_index_forward find_by_pages project].each do |meth|
         # Return a criteria chain in response to a method that will begin or end a chain. For more information,
         # see Dynamoid::Criteria::Chain.
         #

lib/dynamoid/criteria/chain.rb

@@ -159,6 +159,11 @@ def find_by_pages(&block)
         pages.each(&block)
       end
 
+      def project(*fields)
+        @project = fields.map(&:to_sym)
+        self
+      end
+
       private
 
       # The actual records referenced by the association.
@@ -190,9 +195,12 @@ def pages
       #
       # @since 3.1.0
       def pages_via_query
-        Enumerator.new do |yielder|
+        Enumerator.new do |y|
           Dynamoid.adapter.query(source.table_name, range_query).each do |items, metadata|
-            yielder.yield items.map { |hash| source.from_database(hash) }, metadata.slice(:last_evaluated_key)
+            page = items.map { |h| source.from_database(h) }
+            options = metadata.slice(:last_evaluated_key)
+
+            y.yield page, options
           end
         end
       end
@@ -203,9 +211,12 @@ def pages_via_query
       #
       # @since 3.1.0
       def pages_via_scan
-        Enumerator.new do |yielder|
+        Enumerator.new do |y|
           Dynamoid.adapter.scan(source.table_name, scan_query, scan_opts).each do |items, metadata|
-            yielder.yield(items.map { |hash| source.from_database(hash) }, metadata.slice(:last_evaluated_key))
+            page = items.map { |h| source.from_database(h) }
+            options = metadata.slice(:last_evaluated_key)
+
+            y.yield page, options
           end
         end
       end
@@ -368,13 +379,16 @@ def start_key
 
       def query_opts
         opts = {}
+        # Don't specify select = ALL_ATTRIBUTES option explicitly because it's
+        # already a default value of Select statement. Explicite Select value
+        # conflicts with AttributesToGet statement (project option).
         opts[:index_name] = @key_fields_detector.index_name if @key_fields_detector.index_name
-        opts[:select] = 'ALL_ATTRIBUTES'
         opts[:record_limit] = @record_limit if @record_limit
         opts[:scan_limit] = @scan_limit if @scan_limit
         opts[:batch_size] = @batch_size if @batch_size
         opts[:exclusive_start_key] = start_key if @start
         opts[:scan_index_forward] = @scan_index_forward
+        opts[:project] = @project
         opts
       end
 
@@ -398,6 +412,7 @@ def scan_opts
         opts[:batch_size] = @batch_size if @batch_size
         opts[:exclusive_start_key] = start_key if @start
         opts[:consistent_read] = true if @consistent_read
+        opts[:project] = @project
         opts
       end
     end

spec/dynamoid/criteria/chain_spec.rb

@@ -1511,6 +1511,45 @@ def request_params
     end
   end
 
+  describe '#project' do
+    let(:model) do
+      new_class do
+        field :name
+        field :age, :integer
+      end
+    end
+
+    it 'loads only specified attributes' do
+      model.create(name: 'Alex', age: 21)
+      obj, = model.project(:age).to_a
+
+      expect(obj.age).to eq 21
+
+      expect(obj.id).to eq nil
+      expect(obj.name).to eq nil
+    end
+
+    it 'works with Scan' do
+      model.create(name: 'Alex', age: 21)
+
+      chain = Dynamoid::Criteria::Chain.new(model)
+      expect(chain).to receive(:pages_via_scan).and_call_original
+
+      obj, = chain.project(:age).to_a
+      expect(obj.attributes).to eq(age: 21)
+    end
+
+    it 'works with Query' do
+      obj = model.create(name: 'Alex', age: 21)
+
+      chain = Dynamoid::Criteria::Chain.new(model)
+      expect(chain).to receive(:pages_via_query).and_call_original
+
+      obj_loaded, = chain.where(id: obj.id).project(:age).to_a
+      expect(obj_loaded.attributes).to eq(age: 21)
+    end
+  end
+
   describe 'User' do
     let(:chain) { described_class.new(User) }