For RUBY ON RAILS DEVS who use app/services - this automatically creates the modules and class which is dependant on the file name. put it inside your init.lua file before the return.

gistfile1.txt

-- Auto-generate Ruby module/class skeleton for app/services/**.rb
local function camelize(s)
  -- foo_bar -> FooBar
  return (s:gsub("(%a)([%w_]*)", function(first, rest)
    return first:upper() .. rest
  end):gsub("_(%a)", function(c)
    return c:upper()
  end))
end

local function split_path(p)
  local t = {}
  for part in p:gmatch("[^/]+") do
    table.insert(t, part)
  end
  return t
end

local function is_empty_buffer(bufnr)
  if vim.api.nvim_buf_line_count(bufnr) ~= 1 then return false end
  return vim.api.nvim_buf_get_lines(bufnr, 0, 1, false)[1] == ""
end

local function insert_ruby_services_skeleton()
  local bufnr = vim.api.nvim_get_current_buf()
  if not is_empty_buffer(bufnr) then return end

  local fullpath = vim.fn.expand("%:p")
  -- match anything after app/services/ and ending in .rb
  local rel = fullpath:match("/app/services/(.+)%.rb$")
  if not rel then return end

  local parts = split_path(rel)
  if #parts == 0 then return end

  local filename = parts[#parts]:gsub("%.rb$", "")
  parts[#parts] = nil

  local modules = {}
  for _, p in ipairs(parts) do
    table.insert(modules, camelize(p))
  end

  local class_name = camelize(filename)

  local lines = {}
  local indent = ""

  for _, m in ipairs(modules) do
    table.insert(lines, indent .. "module " .. m)
    indent = indent .. "  "
  end

  table.insert(lines, indent .. "class " .. class_name)
  table.insert(lines, indent .. "end")

  for i = #modules, 1, -1 do
    indent = indent:sub(1, -3) -- remove two spaces
    table.insert(lines, indent .. "end")
  end

  vim.api.nvim_buf_set_lines(bufnr, 0, -1, false, lines)

  local cursor_line = (#modules) + 1 -- line with "class ..."
  vim.api.nvim_win_set_cursor(0, { cursor_line + 1, (#modules * 2) }) -- inside class, indentation
end

vim.api.nvim_create_autocmd({ "BufNewFile" }, {
  pattern = { "*.rb" },
  callback = insert_ruby_services_skeleton,
})