Add comment model and its tests

spec/factories/comments.rb

@@ -0,0 +1,8 @@
+FactoryBot.define do
+  factory :comment do
+    body { "MyText" }
+    user
+    post
+    parent { nil }
+  end
+end

spec/models/comment_spec.rb

@@ -0,0 +1,50 @@
+require 'rails_helper'
+
+RSpec.describe Comment, type: :model do
+  let(:comment) { FactoryBot.build(:comment) }
+
+  it 'should be valid' do
+    expect(comment).to be_valid
+  end
+
+  it 'has a body' do
+    nil_body = FactoryBot.build(:comment, body: nil)
+    empty_body = FactoryBot.build(:comment, body: '')
+
+    expect(nil_body).to be_invalid
+    expect(empty_body).to be_invalid
+  end
+
+  it 'has a body with 4 or more characters' do
+    short_body = FactoryBot.build(:comment, body: 'a' * 3)
+
+    expect(short_body).to be_invalid
+  end
+
+  it 'can have no parent' do
+    no_parent = FactoryBot.build(:comment, parent: nil)
+
+    expect(no_parent).to be_valid
+    expect(no_parent.parent).to be_nil
+  end
+
+  it 'can have a parent' do
+    parent = FactoryBot.build(:comment)
+    child = FactoryBot.build(:comment, parent: parent)
+
+    expect(child).to be_valid
+    expect(child.parent).to eq(parent)
+  end
+
+  it 'can have no children' do
+    expect(comment.children).to be_empty
+  end
+
+  it 'can have 1+ children' do
+    parent = FactoryBot.create(:comment)
+    child1 = FactoryBot.create(:comment, parent: parent)
+    child2 = FactoryBot.create(:comment, parent: parent)
+
+    expect(parent.children.length).to eq(2)
+  end
+end