create initial records
get initial records
[
  { id: 'alfon', name: 'alfonsus' },
  { id: 'fuma', name: 'fuma' },
  { id: 'joulev', name: 'joulev' }
]
[
  {
    attachmentUrl: null,
    authorId: 'fuma',
    content: 'hello world',
    id: '1',
    relyTo: null
  },
  {
    attachmentUrl: 'attachment-1',
    authorId: 'joulev',
    content: 'default content.',
    id: '2',
    relyTo: '1'
  },
  {
    attachmentUrl: null,
    authorId: 'alfon',
    content: 'hehe',
    id: '3',
    relyTo: null
  }
]
[
  { data: Uint8Array(4) [ 1, 2, 3, 4 ], id: '1', url: 'attachment-1' }
]
delete alfon, his posts should also be deleted
[
  {
    attachmentUrl: null,
    authorId: 'fuma',
    content: 'hello world',
    id: '1',
    relyTo: null
  },
  {
    attachmentUrl: 'attachment-1',
    authorId: 'joulev',
    content: 'default content.',
    id: '2',
    relyTo: '1'
  }
]
update attachment url of post 2, attachment url should also be updated
[
  {
    data: Uint8Array(4) [ 1, 2, 3, 4 ],
    id: '1',
    url: 'attachment-1-updated'
  }
]
delete post, attachment should also be deleted
[]