diff --git a/.ruby-version b/.ruby-version
new file mode 100644
index 0000000..aedc15b
--- /dev/null
+++ b/.ruby-version
@@ -0,0 +1 @@
+2.5.3
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 608e79b..672f85b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,8 @@
 
 ## master
 
+- [PR#30](https://github.com/DmitryTsepelev/graphql-ruby-fragment_cache/pull/30) Add union support ([@DmitryTsepelev][])
+
 ## 1.0.3 (2020-08-31)
 
 - [PR#29](https://github.com/DmitryTsepelev/graphql-ruby-fragment_cache/pull/29) Cache result JSON instead of connection objects ([@DmitryTsepelev][])
diff --git a/lib/graphql/fragment_cache/cache_key_builder.rb b/lib/graphql/fragment_cache/cache_key_builder.rb
index 9db7364..8ca2b1e 100644
--- a/lib/graphql/fragment_cache/cache_key_builder.rb
+++ b/lib/graphql/fragment_cache/cache_key_builder.rb
@@ -32,9 +32,23 @@ def alias?(val)
       end
 
       refine ::GraphQL::Execution::Lookahead do
+        using RubyNext
+
         def selection_with_alias(name, **kwargs)
-          return selection(name, **kwargs) if selects?(name, **kwargs)
-          alias_selection(name, **kwargs)
+          # In case of union we have to pass a type of object explicitly
+          # More info https://github.com/rmosolgo/graphql-ruby/pull/3007
+          if @selected_type.kind.union?
+            # TODO: we need to guess a type of an object at path to pass it
+            kwargs[:selected_type] = @query.context.namespace(:interpreter)[:current_object].class
+          end
+
+          selection(name, **kwargs).then do |next_selection|
+            if next_selection.is_a?(GraphQL::Execution::Lookahead::NullLookahead)
+              alias_selection(name, **kwargs)
+            else
+              next_selection
+            end
+          end
         end
 
         def alias_selection(name, selected_type: @selected_type, arguments: nil)
@@ -48,7 +62,9 @@ def alias_selection(name, selected_type: @selected_type, arguments: nil)
           # From https://github.com/rmosolgo/graphql-ruby/blob/1a9a20f3da629e63ea8e5ee8400be82218f9edc3/lib/graphql/execution/lookahead.rb#L91
           next_field_defn = get_class_based_field(selected_type, next_field_name)
 
-          alias_selections[name] =
+          alias_name = "#{name}_#{selected_type.name}"
+
+          alias_selections[alias_name] =
             if next_field_defn
               next_nodes = []
               arguments = @query.arguments_for(alias_node, next_field_defn)
diff --git a/spec/graphql/fragment_cache/cache_key_builder_spec.rb b/spec/graphql/fragment_cache/cache_key_builder_spec.rb
index 5d8fe80..aad8a42 100644
--- a/spec/graphql/fragment_cache/cache_key_builder_spec.rb
+++ b/spec/graphql/fragment_cache/cache_key_builder_spec.rb
@@ -57,6 +57,7 @@
       GQL
     end
 
+
     specify { is_expected.to eq "schema_key/cachedPost(id:#{id})[id.title.author[id.name]]" }
   end
 
@@ -77,7 +78,6 @@
     end
 
     let(:path) { ["cachedPostByInput"] }
-
     let(:variables) { {inputWithId: {id: id, intArg: 42}} }
 
     specify { is_expected.to eq "schema_key/cachedPostByInput(input_with_id:{id:#{id},int_arg:42})[id.title.author[id.name]]" }
@@ -147,6 +147,7 @@
       GQL
     end
 
+
     specify { is_expected.to eq "schema_key/cachedPost(id:#{id})[id.title.author[id.name]]" }
 
     context "when nested fragment is used" do
@@ -216,7 +217,6 @@
         }
       GQL
     end
-
     let(:path) { ["posts", 0, "cachedTitle"] }
 
     specify { is_expected.to eq "schema_key/posts/0/cachedTitle[]" }
@@ -265,4 +265,73 @@
 
     specify { is_expected.to eq "schema_key/post(id:1)/cachedAuthor[name]" }
   end
+
+  context "when query has union type" do
+    let(:path) { ["lastActivity", "cachedAvatarUrl"] }
+
+    let(:query) do
+      <<~GQL
+        query getLastActivity {
+          lastActivity {
+            ...on PostType {
+              id
+              cachedAvatarUrl
+            }
+            ...on UserType {
+              id
+              cachedAvatarUrl
+            }
+          }
+        }
+      GQL
+    end
+
+    specify { is_expected.to eq "schema_key/lastActivity/cachedAvatarUrl[]" }
+
+    context "when array of union typed objects is returned" do
+      let(:query) do
+        <<~GQL
+          query getFeed {
+            feed {
+              ...on PostType {
+                id
+                cachedAvatarUrl
+              }
+              ...on UserType {
+                id
+                cachedAvatarUrl
+              }
+            }
+          }
+        GQL
+      end
+
+      let(:path) { ["feed", 0, "cachedAvatarUrl"] }
+
+      specify { is_expected.to eq "schema_key/feed/0/cachedAvatarUrl[]" }
+
+      context "when cached field has alias" do
+        let(:query) do
+          <<~GQL
+            query getFeed {
+              feed {
+                ...on PostType {
+                  id
+                  avatarUrl: cachedAvatarUrl
+                }
+                ...on UserType {
+                  id
+                  avatarUrl: cachedAvatarUrl
+                }
+              }
+            }
+          GQL
+        end
+
+        let(:path) { ["feed", 0, "avatarUrl"] }
+
+        specify { is_expected.to eq "schema_key/feed/0/cachedAvatarUrl[]" }
+      end
+    end
+  end
 end
diff --git a/spec/graphql/fragment_cache/object_helpers_spec.rb b/spec/graphql/fragment_cache/object_helpers_spec.rb
index fff914d..9fc3bd3 100644
--- a/spec/graphql/fragment_cache/object_helpers_spec.rb
+++ b/spec/graphql/fragment_cache/object_helpers_spec.rb
@@ -577,4 +577,124 @@ def post(id:, expires_in: nil)
       expect(::Post).not_to have_received(:all)
     end
   end
+
+  describe "union caching" do
+    let!(:post) { Post.create(id: 1, title: "Post #1") }
+    let!(:user) { User.create(id: 2, name: "User #2") }
+
+    let(:schema) do
+      build_schema do
+        query(
+          Class.new(Types::Query) {
+            field :last_activity, Types::Activity, null: false
+
+            define_method(:last_activity, -> { ::Post.find(1) })
+          }
+        )
+      end
+    end
+
+    let(:query) do
+      <<~GQL
+        query getLastActivity {
+          lastActivity {
+            ...on PostType {
+              id
+              cachedAvatarUrl
+            }
+            ...on UserType {
+              id
+              cachedAvatarUrl
+            }
+          }
+        }
+      GQL
+    end
+
+    it "returns cached data" do
+      expect(execute_query.dig("data")).to eq(
+        "lastActivity" =>
+          {"cachedAvatarUrl" => "http://example.com/img/posts/#{post.id}", "id" => post.id.to_s}
+      )
+    end
+
+    context "when unions are nested" do
+      let(:query) do
+        <<~GQL
+          query getLastActivity {
+            lastActivity {
+              ...on PostType {
+                id
+                relatedActivity {
+                  ...on PostType {
+                    id
+                    cachedAvatarUrl
+                  }
+                  ...on UserType {
+                    id
+                    cachedAvatarUrl
+                  }
+                }
+              }
+              ...on UserType {
+                id
+              }
+            }
+          }
+        GQL
+      end
+
+      it "returns cached data" do
+        expect(execute_query.dig("data")).to eq(
+          "lastActivity" => {
+            "id" => "1",
+            "relatedActivity" => {
+              "cachedAvatarUrl" => "http://example.com/img/posts/#{user.id}",
+              "id" => user.id.to_s
+            }
+          }
+        )
+      end
+    end
+
+    context "when array of union typed objects is returned" do
+      let(:schema) do
+        build_schema do
+          query(
+            Class.new(Types::Query) {
+              field :feed, [Types::Activity], null: false
+
+              define_method(:feed, -> { ::Post.all + ::User.all })
+            }
+          )
+        end
+      end
+
+      let(:query) do
+        <<~GQL
+          query getFeed {
+            feed {
+              ...on PostType {
+                id
+                cachedAvatarUrl
+              }
+              ...on UserType {
+                id
+                cachedAvatarUrl
+              }
+            }
+          }
+        GQL
+      end
+
+      it "returns cached data" do
+        expect(execute_query.dig("data")).to eq(
+          "feed" => [
+            {"cachedAvatarUrl" => "http://example.com/img/posts/#{post.id}", "id" => post.id.to_s},
+            {"cachedAvatarUrl" => "http://example.com/img/users/#{user.id}", "id" => user.id.to_s}
+          ]
+        )
+      end
+    end
+  end
 end
diff --git a/spec/graphql/fragment_cache/rails/cache_key_builder_spec.rb b/spec/graphql/fragment_cache/rails/cache_key_builder_spec.rb
index 5ce6c47..e93fee1 100644
--- a/spec/graphql/fragment_cache/rails/cache_key_builder_spec.rb
+++ b/spec/graphql/fragment_cache/rails/cache_key_builder_spec.rb
@@ -26,6 +26,7 @@
 
   let(:object) { Post.find(42) }
   let(:query_obj) { GraphQL::Query.new(schema, query, variables: variables) }
+  let(:selected_type) { Types::Post }
 
   # Make cache keys raw for easier debugging
   let(:schema_cache_key) { "schema_key" }
@@ -34,7 +35,11 @@
     allow(Digest::SHA1).to receive(:hexdigest) { |val| val }
   end
 
-  subject { described_class.call(object: object, query: query_obj, path: path) }
+  subject do
+    described_class.call(
+      object: object, query: query_obj, selected_type: selected_type, path: path
+    )
+  end
 
   it "uses Cache.expand_cache_key" do
     allow(ActiveSupport::Cache).to receive(:expand_cache_key).with(object) { "as:cache:key" }
diff --git a/spec/support/models/post.rb b/spec/support/models/post.rb
index 9d504be..6625d56 100644
--- a/spec/support/models/post.rb
+++ b/spec/support/models/post.rb
@@ -4,7 +4,7 @@ class Post
   class << self
     def find(id)
       store.fetch(id.to_i) do
-        author = User.new(id: id, name: "User ##{id}")
+        author = User.fetch(id)
         new(id: id, title: "Post ##{id}", author: author)
       end
     end
diff --git a/spec/support/models/user.rb b/spec/support/models/user.rb
index 27b84d4..566bd0a 100644
--- a/spec/support/models/user.rb
+++ b/spec/support/models/user.rb
@@ -4,6 +4,26 @@ class User
   attr_reader :id
   attr_accessor :name
 
+  class << self
+    def fetch(id)
+      store.fetch(id.to_i) do
+        new(id: id, name: "User ##{id}")
+      end
+    end
+
+    def all
+      @store.values
+    end
+
+    def create(id:, **attributes)
+      store[id] = new(id: id, **attributes)
+    end
+
+    def store
+      @store ||= {}
+    end
+  end
+
   def initialize(id:, name:)
     @id = id
     @name = name
diff --git a/spec/support/test_schema.rb b/spec/support/test_schema.rb
index 2f2a2b8..2190d84 100644
--- a/spec/support/test_schema.rb
+++ b/spec/support/test_schema.rb
@@ -10,6 +10,14 @@ class User < Base
 
     field :id, ID, null: false
     field :name, String, null: false
+    field :cached_avatar_url, String, null: true
+
+    def cached_avatar_url
+      cache_fragment { "http://example.com/img/users/#{object.id}" }
+    end
+  end
+
+  class Activity < GraphQL::Schema::Union
   end
 
   class Post < Base
@@ -18,14 +26,36 @@ class Post < Base
     field :id, ID, null: false
     field :title, String, null: false
     field :cached_title, String, null: false, cache_fragment: true, method: :title
+    field :cached_avatar_url, String, null: true
     field :author, User, null: false
     field :cached_author, User, null: false
+    field :related_activity, Activity, null: true
 
     field :meta, String, null: true
 
     def cached_author
       cache_fragment { object.author }
     end
+
+    def cached_avatar_url
+      cache_fragment { "http://example.com/img/posts/#{object.id}" }
+    end
+
+    def related_activity
+      ::User.all.last
+    end
+  end
+
+  class Activity < GraphQL::Schema::Union
+    graphql_name "ActivityType"
+
+    description "Represents chat message"
+
+    possible_types Post, User
+
+    def self.resolve_type(object, _context)
+      Kernel.const_get("Types::#{object.class.name}")
+    end
   end
 
   class PostInput < GraphQL::Schema::InputObject
@@ -59,6 +89,10 @@ class Query < Base
       argument :complex_post_input, ComplexPostInput, required: true
     end
 
+    field :feed, [Activity], null: false
+
+    field :last_activity, Activity, null: false
+
     def post(id:)
       ::Post.find(id)
     end
@@ -67,6 +101,14 @@ def posts
       ::Post.all
     end
 
+    def feed
+      ::Post.all + ::User.all
+    end
+
+    def last_activity
+      ::Post.find(1)
+    end
+
     def cached_post(id:)
       cache_fragment { ::Post.find(id) }
     end