diff --git a/app/models/concerns/flippable.rb b/app/models/concerns/feature_gate.rb
similarity index 82%
rename from app/models/concerns/flippable.rb
rename to app/models/concerns/feature_gate.rb
index 341501e8250..5db64fe82c4 100644
--- a/app/models/concerns/flippable.rb
+++ b/app/models/concerns/feature_gate.rb
@@ -1,4 +1,4 @@
-module Flippable
+module FeatureGate
   def flipper_id
     return nil if new_record?
 
diff --git a/app/models/user.rb b/app/models/user.rb
index bcce260ab08..e08096284ef 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -11,7 +11,7 @@ class User < ActiveRecord::Base
   include CaseSensitivity
   include TokenAuthenticatable
   include IgnorableColumn
-  include Flippable
+  include FeatureGate
 
   DEFAULT_NOTIFICATION_LEVEL = :participating
 
diff --git a/doc/api/features.md b/doc/api/features.md
index 0ca2e637614..a3bf5d018a7 100644
--- a/doc/api/features.md
+++ b/doc/api/features.md
@@ -61,6 +61,8 @@ POST /features/:name
 | `flipper_group` | string | no | A Flipper group name |
 | `user` | string | no | A GitLab username |
 
+Note that `flipper_group` and `user` are mutually exclusive.
+
 ```bash
 curl --data "value=30" --header "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" https://gitlab.example.com/api/v4/features/new_library
 ```
diff --git a/lib/api/features.rb b/lib/api/features.rb
index 4e10e36fce9..e426bc050eb 100644
--- a/lib/api/features.rb
+++ b/lib/api/features.rb
@@ -42,6 +42,7 @@ module API
         requires :value, type: String, desc: '`true` or `false` to enable/disable, an integer for percentage of time'
         optional :flipper_group, type: String, desc: 'A Flipper group name'
         optional :user, type: String, desc: 'A GitLab username'
+        mutually_exclusive :flipper_group, :user
       end
       post ':name' do
         feature = Feature.get(params[:name])
diff --git a/spec/models/concerns/feature_gate_spec.rb b/spec/models/concerns/feature_gate_spec.rb
new file mode 100644
index 00000000000..3f601243245
--- /dev/null
+++ b/spec/models/concerns/feature_gate_spec.rb
@@ -0,0 +1,19 @@
+require 'spec_helper'
+
+describe FeatureGate do
+  describe 'User' do
+    describe '#flipper_id' do
+      context 'when user is not persisted' do
+        let(:user) { build(:user) }
+
+        it { expect(user.flipper_id).to be_nil }
+      end
+
+      context 'when user is persisted' do
+        let(:user) { create(:user) }
+
+        it { expect(user.flipper_id).to eq "User:#{user.id}" }
+      end
+    end
+  end
+end
diff --git a/spec/models/user_spec.rb b/spec/models/user_spec.rb
index 05ba887c51f..8e895ec6634 100644
--- a/spec/models/user_spec.rb
+++ b/spec/models/user_spec.rb
@@ -430,20 +430,6 @@ describe User, models: true do
     end
   end
 
-  describe '#flipper_id' do
-    context 'when user is not persisted' do
-      let(:user) { build(:user) }
-
-      it { expect(user.flipper_id).to be_nil }
-    end
-
-    context 'when user is persisted' do
-      let(:user) { create(:user) }
-
-      it { expect(user.flipper_id).to eq "User:#{user.id}" }
-    end
-  end
-
   describe '#generate_password' do
     it "does not generate password by default" do
       user = create(:user, password: 'abcdefghe')