Default value + Callable syntax

CHANGELOG.md

@@ -10,7 +10,8 @@ that you can set version constraints properly.
 
 #### [Unreleased](https://github.com/exAspArk/batch-loader/compare/v1.0.4...HEAD)
 
-* WIP
+* `Added`: `default_value` override option.
+* `Added`: `loader.call {}` block syntax, for memoizing repeat calls to the same item.
 
 #### [v1.0.4](https://github.com/exAspArk/batch-loader/compare/v1.0.3...v1.0.4)
 

README.md

@@ -13,7 +13,7 @@ This gem provides a generic lazy batching mechanism to avoid N+1 DB queries, HTT
 * [Highlights](#highlights)
 * [Usage](#usage)
   * [Why?](#why)
-  * [Basic example](#basic-example)
+  * [Basic examples](#basic-examples)
   * [How it works](#how-it-works)
   * [RESTful API example](#restful-api-example)
   * [GraphQL example](#graphql-example)
@@ -97,7 +97,7 @@ puts users                     #      Users
 
 But the problem here is that `load_posts` now depends on the child association and knows that it has to preload data for future use. And it'll do it every time, even if it's not necessary. Can we do better? Sure!
 
-### Basic example
+### Basic examples
 
 With `BatchLoader` we can rewrite the code above:
 
@@ -125,6 +125,51 @@ puts users                     #      Users      SELECT * FROM users WHERE id IN
 
 As we can see, batching is isolated and described right in a place where it's needed.
 
+For batches where there is no item in response to a call, we normally return nil. However, you can use `default_value:` to return something else instead. This is particularly useful for 1:Many relationships, where you
+
+```ruby
+def load_posts(ids)
+  Post.where(id: ids)
+end
+
+def load_user(post)
+  BatchLoader.for(post.user_id).batch(default_value: NullUser.new) do |user_ids, loader|
+    User.where(id: user_ids).each { |user| loader.call(user.id, user) }
+  end
+end
+
+posts = load_posts([1, 2, 3])
+
+
+users = posts.map do |post|
+  load_user(post)
+end
+
+puts users
+```
+
+For batches where the value is some kind of collection, such as an Array or Hash, `loader` also supports being called with a block, which yields the _current_ value, and returns the _next_ value. This is extremely useful for 1:Many relationships:
+
+```ruby
+def load_users(ids)
+  User.where(id: ids)
+end
+
+def load_comments(user)
+  BatchLoader.for(user.id).batch(default_value: []) do |comment_ids, loader|
+    Comment.where(user_id: user_ids).each do |comment|
+      loader.call(user.id) { |memo| memo.push(comment) }
+    end
+  end
+end
+
+users = load_users([1, 2, 3])
+
+comments = users.map do |user|
+  load_comments(user)
+end
+```
+
 ### How it works
 
 In general, `BatchLoader` returns a lazy object. Each lazy object knows which data it needs to load and how to batch the query. As soon as you need to use the lazy objects, they will be automatically loaded once without N+1 queries.

lib/batch_loader.rb

@@ -11,6 +11,7 @@ class BatchLoader
   IMPLEMENTED_INSTANCE_METHODS = %i[object_id __id__ __send__ singleton_method_added __sync respond_to? batch inspect].freeze
   REPLACABLE_INSTANCE_METHODS = %i[batch inspect].freeze
   LEFT_INSTANCE_METHODS = (IMPLEMENTED_INSTANCE_METHODS - REPLACABLE_INSTANCE_METHODS).freeze
+  NULL_VALUE = :batch_loader_null
 
   NoBatchError = Class.new(StandardError)
 
@@ -22,7 +23,8 @@ def initialize(item:)
     @item = item
   end
 
-  def batch(cache: true, &batch_block)
+  def batch(default_value: nil, cache: true, &batch_block)
+    @default_value = default_value
     @cache = cache
     @batch_block = batch_block
     __executor_proxy.add(item: @item)
@@ -75,12 +77,20 @@ def __ensure_batched
     return if __executor_proxy.value_loaded?(item: @item)
 
     items = __executor_proxy.list_items
-    loader = ->(item, value) { __executor_proxy.load(item: item, value: value) }
+    loader =  -> (item, value = NULL_VALUE, &block) {
+      if block
+        raise ArgumentError, "Please pass a value or a block, not both" if value != NULL_VALUE
+        next_value = block.call(__executor_proxy.loaded_value(item: item))
+      else
+        next_value = value
+      end
+      __executor_proxy.load(item: item, value: next_value)
+    }
 
     @batch_block.call(items, loader)
     items.each do |item|
       next if __executor_proxy.value_loaded?(item: item)
-      loader.call(item, nil) # use "nil" for not loaded item after succesfull batching
+      loader.call(item, @default_value)
     end
     __executor_proxy.delete(items: items)
   end
@@ -109,7 +119,7 @@ def __purge_cache
   def __executor_proxy
     @__executor_proxy ||= begin
       raise NoBatchError.new("Please provide a batch block first") unless @batch_block
-      BatchLoader::ExecutorProxy.new(&@batch_block)
+      BatchLoader::ExecutorProxy.new(@default_value, &@batch_block)
     end
   end
 

lib/batch_loader/executor_proxy.rb

@@ -4,9 +4,10 @@
 
 class BatchLoader
   class ExecutorProxy
-    attr_reader :block, :global_executor
+    attr_reader :default_value, :block, :global_executor
 
-    def initialize(&block)
+    def initialize(default_value, &block)
+      @default_value = default_value
       @block = block
       @block_hash_key = block.source_location
       @global_executor = BatchLoader::Executor.ensure_current
@@ -29,7 +30,11 @@ def load(item:, value:)
     end
 
     def loaded_value(item:)
-      loaded[item]
+      if value_loaded?(item: item)
+        loaded[item]
+      else
+        @default_value.dup
+      end
     end
 
     def value_loaded?(item:)

spec/batch_loader_spec.rb

@@ -98,6 +98,38 @@
 
       expect(lazy).to eq(2)
     end
+
+    it 'supports alternative default values' do
+      lazy = BatchLoader.for(1).batch(default_value: 123) do |nums, loader|
+        # No-op, so default is returned
+      end
+
+      expect(lazy).to eq(123)
+    end
+
+    it 'supports memoizing repeated calls to the same item, via a block' do
+      lazy = BatchLoader.for(1).batch(default_value: []) do |nums, loader|
+        nums.each do |num|
+          loader.call(num) { |memo| memo.push(num) }
+          loader.call(num) { |memo| memo.push(num + 1) }
+          loader.call(num) { |memo| memo.push(num + 2) }
+        end
+      end
+
+      expect(lazy).to eq([1,2,3])
+    end
+
+    context "called with block and value syntax" do
+      it 'raises ArgumentError' do
+        lazy = BatchLoader.for(1).batch(default_value: {}) do |nums, loader|
+          nums.each do |num|
+            loader.call(num, "one value") { "too many values" }
+          end
+        end
+
+        expect { lazy.sync }.to raise_error(ArgumentError)
+      end
+    end
   end
 
   describe '#inspect' do