Assign plan_id when building a new plan limit

Using `PlanLimits.new` fails because the `plan_id` FK is not set.
We were also directing users to use `Plan.default.limits` which can be
nil if there are no limit records.

app/models/plan.rb

@@ -27,7 +27,7 @@ class Plan < ApplicationRecord
   end
 
   def actual_limits
-    self.limits || PlanLimits.new
+    self.limits || self.build_limits
   end
 
   def default?

changelogs/unreleased/fix-update-plan-limits-functionality.yml

+---
+title: Assign plan_id when building a new plan limit
+merge_request: 34845
+author:
+type: fixed

doc/administration/instance_limits.md

@@ -77,10 +77,10 @@ To set this limit on a self-managed installation, run the following in the
 # Plan.default.create_limits!
 
 # For project webhooks
-Plan.default.limits.update!(project_hooks: 100)
+Plan.default.actual_limits.update!(project_hooks: 100)
 
 # For group webhooks
-Plan.default.limits.update!(group_hooks: 100)
+Plan.default.actual_limits.update!(group_hooks: 100)
 ```
 
 NOTE: **Note:** Set the limit to `0` to disable it.
@@ -115,7 +115,7 @@ To set this limit on a self-managed installation, run the following in the
 # If limits don't exist for the default plan, you can create one with:
 # Plan.default.create_limits!
 
-Plan.default.limits.update!(offset_pagination_limit: 10000)
+Plan.default.actual_limits.update!(offset_pagination_limit: 10000)
 ```
 
 - **Default offset pagination limit:** 50000
@@ -149,7 +149,7 @@ To set this limit on a self-managed installation, run the following in the
 # If limits don't exist for the default plan, you can create one with:
 # Plan.default.create_limits!
 
-Plan.default.limits.update!(ci_active_jobs: 500)
+Plan.default.actual_limits.update!(ci_active_jobs: 500)
 ```
 
 NOTE: **Note:** Set the limit to `0` to disable it.
@@ -171,7 +171,7 @@ To set this limit on a self-managed installation, run the following in the
 [GitLab Rails console](troubleshooting/debug.md#starting-a-rails-console-session):
 
 ```ruby
-Plan.default.limits.update!(ci_project_subscriptions: 500)
+Plan.default.actual_limits.update!(ci_project_subscriptions: 500)
 ```
 
 NOTE: **Note:** Set the limit to `0` to disable it.
@@ -196,7 +196,7 @@ To set this limit on a self-managed installation, run the following in the
 [GitLab Rails console](troubleshooting/debug.md#starting-a-rails-console-session):
 
 ```ruby
-Plan.default.limits.update!(ci_pipeline_schedules: 100)
+Plan.default.actual_limits.update!(ci_pipeline_schedules: 100)
 ```
 
 ### Number of instance level variables
@@ -214,7 +214,7 @@ To update this limit to a new value on a self-managed installation, run the foll
 [GitLab Rails console](troubleshooting/debug.md#starting-a-rails-console-session):
 
 ```ruby
-Plan.default.limits.update!(ci_instance_level_variables: 30)
+Plan.default.actual_limits.update!(ci_instance_level_variables: 30)
 ```
 
 ## Instance monitoring and metrics

spec/models/plan_spec.rb

@@ -14,4 +14,16 @@ describe Plan do
       end
     end
   end
+
+  context 'when updating plan limits' do
+    let(:plan) { described_class.default }
+
+    it { expect(plan).to be_persisted }
+
+    it { expect(plan.actual_limits).not_to be_persisted }
+
+    it 'successfully updates the limits' do
+      expect(plan.actual_limits.update!(ci_instance_level_variables: 100)).to be_truthy
+    end
+  end
 end