db.rb

def db
  @db ||= SQLite3::Database.new(path.to_s).tap do |db|
    db.results_as_hash = true

    db.instance_eval do
      def tables
        rows = execute "SELECT name FROM sqlite_master WHERE type='table';"
        rows.map{|r| r['name']}
      end

      def column_syms_for_table(table)
        r = prepare "SELECT * FROM #{table};"
        r.columns.map &:intern
      end

      def method_missing(m, *args, &block)
        super unless tables.include? m.to_s

        columns = column_syms_for_table m
        Table.new \
          name:    m.to_s,
          columns: columns,
          db:      self
      end
    end
  end
end

table.rb

class Table
  virtus :name, :columns, :db
  # `name`    - table name str
  # `columns` - an array of symbols
  # `db`      - SQLite3::Database instance

  def insert(hash)
    sql  = "INSERT INTO #{name} ( #{comma_sep_column_names} ) VALUES ( #{val_question_marks} )"
    vals = vals_for_hash hash
    db.execute sql, *vals
  end

  def count
    db.execute("SELECT COUNT(*) FROM #{name};")[0][0]
  end

private

  def comma_sep_column_names
    columns.join(', ')
  end

  def val_question_marks
    comma_sep_column_names
    .gsub(/\w+/, '?')
  end

  def vals_for_hash(hash)
    columns.map {|cname| hash[cname] }
  end
end