JSON update has no means to auto-create an updated nested map/array

[Gerold103]

There are cases when a user has a map inside tuple's data somewhere, but it is not always created from the beginning. For example, want to create the map on its first update, when a first key is set. Or worse - the map might be somewhere in-place and empty, or sometimes be not here at all. Easily can happen if the map is a nullable field. However it is not possible to create the map + add a key to it atomically. See example (it is about updating tuples directly, but same is true for space updates):

map = setmetatable({}, {__serialize='map'})
t1 = box.tuple.new({map})
t2 = box.tuple.new({nil})
t1:update({{'=', '[1].key', 123}})
t2:update({{'=', '[1].key', 123}})

tarantool> t1:update({{'=', '[1].key', 123}})
---
- [{'key': 123}]
...

tarantool> t2:update({{'=', '[1].key', 123}})
---
- error: Field ''[1].key'' was not found in the tuple
...

Attempt to add a new first operation {'=', '[1]', {}} would loose data if the map was already here and wasn't empty.

As a workaround the user has to get the tuple and either update it manually + make a replace, or add an optional {'=', <path>, {}} operation if the map is not created yet.

All the same is true for arrays.

The possible solutions are below.

---

New operations to ensure map/array existence
Add new operations which ensure that there is a map/array created. They are nop, if the map/array are already here, or create them if there is nil. Their opcodes could be { and [. Examples:

t = box.tuple.new({{key1 = 1, map1 = {key2 = 2}}})
t:update({{'{', '[1].map1.map2'}, {'=', '[1].map1.map2.key3', 3}})
res = {{key1 = 1, map1 = {key2 = 2, map2 = {key3 = 3}}}}

t = box.tuple.new({{key1 = 1, map1 = {key2 = 2, map2 = {}}}})
t:update({{'{', '[1].map1.map2'}, {'=', '[1].map1.map2.key3', 3}})
-- Same.

Basically, { works like = {} if the map is not here or it is nop. Same for [ and arrays. The operations fail if the field exists but it is not of the needed type ({ on an existing number or an array or other non-map type). They also fail if their JSON path following fails earlier than on the last token.

---

Create the JSON path parts automatically
If a JSON path following fails during an update, the missing parts are created on fly automatically. Examples:

t = box.tuple.new({{key1 = 1, map1 = {key2 = 2}}})
t:update({{'=', '[1].map1.map2.key3', 3}})
res = {{key1 = 1, map1 = {key2 = 2, map2 = {key3 = 3}}}}

t = box.tuple.new({{key1 = 1, map1 = {key2 = 2}}})
t:update({{'=', '[1].map1.map2.map3.arr1[3].map4.key3', 3}})
res = {{key1 = 1, map1 = {key2 = 2, map2 = {map3 = {arr1 = [nil, nil, {map4 = {key3 = 3}}]}}}}}

The second example demonstrates how multiple missing parts of the path were created. It breaks backward compatibility though. Perhaps could add a new per-operation option like whether it should auto-create the path.

---

Hybrid solution
Add a new operation which ensures a passed path exists. Can either work like the first option + create all the missing parts of the path instead of failing, or could make a new operation which takes the value for the last token:

-- Create all parts of the <path> if they are missing.
-- Set the value in the end of the path to be a map. Or fail if it already exists and is not a map.
tuple:update({{'/', <path>, '{'}})
-- Same, but the value is an array.
tuple:update({{'/', <path>, '['}})

Other tickets about improving JSON updates: #6006, #5214, #5152, #3388.