OpenAPI nullable flag lost for primitive and array fields in Union{Nothing,T}

[kahliburke]

Description

Union{Nothing, T} fields on @json structs correctly detect nullability via process_field_type (line 746 of autodoc.jl), and the nullable flag is set on current_field at line 756. However, for primitive and array field types, current_field is then reassigned to a new Dict returned by create_primitive_field_schema (line 779) or create_array_field_schema (line 775), discarding the nullable flag.

The fix from PR #255 works correctly for custom struct $ref fields (where current_field is mutated in-place), but not for primitive or array types.

Reproducer

using Oxygen
using HTTP

@kwdef struct MyRequest
    name::String
    max_items::Union{Nothing, Int} = nothing          # primitive nullable
    tags::Union{Nothing, Vector{String}} = nothing     # array nullable
end

@oxidize

@post "/test" function(req, body::Json{MyRequest})
    return body.payload
end

serve(port=9999, async=true)

# Fetch schema
resp = HTTP.get("http://localhost:9999/docs/schema")
schema = JSON.parse(String(resp.body))
props = schema["components"]["schemas"]["MyRequest"]["properties"]

# Expected: max_items and tags should have "nullable": true
println(props["max_items"])  # {"type": "integer"} — missing nullable
println(props["tags"])       # {"type": "array", ...} — missing nullable

Root Cause

In convertobject! (src/autodoc.jl):

# Line 755-757: nullable flag is set correctly
if is_nullable
    current_field["nullable"] = true
end

# ...but then at line 775/779, current_field is reassigned to a new Dict:
current_field = create_array_field_schema(...)      # line 775 — new Dict, no nullable
current_field = create_primitive_field_schema(...)   # line 779 — new Dict, no nullable

Only the $ref path (line 769) preserves nullable because it mutates the existing current_field dict rather than replacing it.

Suggested Fix

Preserve the nullable flag after reassignment:

# After the existing if/elseif chain for struct/array/primitive:
if is_nullable
    current_field["nullable"] = true
end

Or pass is_nullable into the helper functions and have them include it in the returned Dict.

Similarly, default values emitted by create_primitive_field_schema via string(p.default) produce "nothing" for Union{Nothing,T} fields with default=nothing, which is incorrect — these should either omit the default or use null.

Environment

• Oxygen v1.10.0
• Julia 1.12