Fix and document an RSpec::Parameterized::TableSyntax edge-case

Signed-off-by: NRémy Coutable <remy@rymai.me>

doc/development/testing_guide/best_practices.md

@@ -358,16 +358,11 @@ range of inputs, might look like this:
 describe "#==" do
   using RSpec::Parameterized::TableSyntax
 
-  let(:project1) { create(:project) }
-  let(:project2) { create(:project) }
   where(:a, :b, :result) do
     1         | 1        | true
     1         | 2        | false
     true      | true     | true
     true      | false    | false
-    project1  | project1 | true
-    project2  | project2 | true
-    project 1 | project2 | false
   end
 
   with_them do
@@ -380,6 +375,11 @@ describe "#==" do
 end
 ```
 
+CAUTION: **Caution:**
+Only use simple values as input in the `where` block. Using procs, stateful
+objects, FactoryBot-created objects etc. can lead to
+[unexpected results](https://github.com/tomykaira/rspec-parameterized/issues/8).
+
 ### Prometheus tests
 
 Prometheus metrics may be preserved from one test run to another. To ensure that metrics are

spec/models/prometheus_metric_spec.rb

@@ -4,7 +4,6 @@ require 'spec_helper'
 
 describe PrometheusMetric do
   subject { build(:prometheus_metric) }
-  let(:other_project) { build(:project) }
 
   it_behaves_like 'having unique enum values'
 
@@ -16,17 +15,17 @@ describe PrometheusMetric do
   describe 'common metrics' do
     using RSpec::Parameterized::TableSyntax
 
-    where(:common, :project, :result) do
-      false | other_project | true
-      false | nil           | false
-      true  | other_project | false
-      true  | nil           | true
+    where(:common, :with_project, :result) do
+      false | true | true
+      false | false | false
+      true  | true | false
+      true  | false | true
     end
 
     with_them do
       before do
         subject.common = common
-        subject.project = project
+        subject.project = with_project ? build(:project) : nil
       end
 
       it { expect(subject.valid?).to eq(result) }