Json-path index as primary key doesn't work

[ArtDu]

tarantool> construct_key(tree.id, tree:select()[1])
---
- - {'age': 3}
...

tarantool> tree:get(construct_key(tree.id, tree:select()[1]))
---
- error: 'Supplied key type of part 0 does not match index part type: expected scalar'
...

after: #52