build_tree_spec.rb

def build_tree(root_id:)
  root_row = db_find(id: root_id)
  root_node =
  {
    id: root_id,
    text: root_row[:text]
  }
  add_children(root_node, db_find_children(parent_id: root_id))
  root_node
end

def add_children(node, child_rows)
  node[:children] = child_rows.map do |cr|
    {
      id: cr[:id],
      text: cr[:text],
      children: add_children(cr, db_find_children(parent_id: cr[:id]))
    }
  end
end

DATABASE = [
  {id: 1, parent_id: nil, text: 'i am root'},
  {id: 2, parent_id: 1, text: '1st child of 1'},
  {id: 3, parent_id: 1, text: '2nd child of 1'},
  {id: 4, parent_id: 2, text: '1st child of 2'}
]

def db_find(id:)
  DATABASE.find { |row| row[:id] == id}
end

def db_find_children(parent_id:)
  DATABASE.find_all { |row| row[:parent_id] == parent_id }.map(&method(:format_node))
end

def format_node(db_row)
  {
    id: db_row[:id],
    text: db_row[:text],
  }
end

describe 'build_tree' do
  context 'depth 1' do
    subject { build_tree(root_id: 4) }
    it { should eq id: 4, text: '1st child of 2', children: [] }
  end

  context 'depth 2' do
    subject { build_tree(root_id: 2) }
    let :expected_tree do
      {
        id: 2,
        text: '1st child of 1',
        children: [
          {
            id: 4,
            text: '1st child of 2',
            children: []
          }
        ]
      }
    end
    it { should eq expected_tree }
  end

  context 'depth 3' do
    subject { build_tree(root_id: 1) }
    let(:expected_tree) do
      {
        id: 1,
        text: 'i am root',
        children: [
          {
            id: 2,
            text: '1st child of 1',
            children: [
              {id: 4, text: '1st child of 2', children: []}
            ]
          },
          {id: 3, text: '2nd child of 1', children: []}
        ]
      }
    end
    it { should eq expected_tree }
  end
end