Move behavior of `read_attribute` to `AttributeSet`

Moved `Builder` to its own file, as it started looking very weird once I
added private methods to the `AttributeSet` class and the `Builder`
class started to grow.

Would like to refactor `fetch_value` to change to

```ruby
self[name].value(&block)
```

But that requires the attributes to know about their name, which they
currently do not.

1 files changed, 56 insertions, 0 deletions

diff --git a/activerecord/test/cases/attribute_set_test.rb b/activerecord/test/cases/attribute_set_test.rb
index 402a611efa..35254b1087 100644
--- a/activerecord/test/cases/attribute_set_test.rb
+++ b/activerecord/test/cases/attribute_set_test.rb
@@ -61,5 +61,61 @@ module ActiveRecord
       assert_equal({ foo: 1, bar: 2.2 }, attributes.to_hash)
       assert_equal({ foo: 1, bar: 2.2 }, attributes.to_h)
     end
+
+    test "known columns are built with uninitialized attributes" do
+      attributes = attributes_with_uninitialized_key
+      assert attributes[:foo].initialized?
+      assert_not attributes[:bar].initialized?
+    end
+
+    test "uninitialized attributes are not included in the attributes hash" do
+      attributes = attributes_with_uninitialized_key
+      assert_equal({ foo: 1 }, attributes.to_hash)
+    end
+
+    test "uninitialized attributes are not included in keys" do
+      attributes = attributes_with_uninitialized_key
+      assert_equal [:foo], attributes.keys
+    end
+
+    test "uninitialized attributes return false for include?" do
+      attributes = attributes_with_uninitialized_key
+      assert attributes.include?(:foo)
+      assert_not attributes.include?(:bar)
+    end
+
+    test "unknown attributes return false for include?" do
+      attributes = attributes_with_uninitialized_key
+      assert_not attributes.include?(:wibble)
+    end
+
+    test "fetch_value returns the value for the given initialized attribute" do
+      builder = AttributeSet::Builder.new(foo: Type::Integer.new, bar: Type::Float.new)
+      attributes = builder.build_from_database(foo: '1.1', bar: '2.2')
+
+      assert_equal 1, attributes.fetch_value(:foo)
+      assert_equal 2.2, attributes.fetch_value(:bar)
+    end
+
+    test "fetch_value returns nil for unknown attributes" do
+      attributes = attributes_with_uninitialized_key
+      assert_nil attributes.fetch_value(:wibble)
+    end
+
+    test "fetch_value uses the given block for uninitialized attributes" do
+      attributes = attributes_with_uninitialized_key
+      value = attributes.fetch_value(:bar) { |n| n.to_s + '!' }
+      assert_equal 'bar!', value
+    end
+
+    test "fetch_value returns nil for uninitialized attributes if no block is given" do
+      attributes = attributes_with_uninitialized_key
+      assert_nil attributes.fetch_value(:bar)
+    end
+
+    def attributes_with_uninitialized_key
+      builder = AttributeSet::Builder.new(foo: Type::Integer.new, bar: Type::Float.new)
+      builder.build_from_database(foo: '1.1')
+    end
   end
 end