Convert custom sources to dictionaries through lower

src/StructTypes.jl

@@ -933,7 +933,8 @@ and respecting various serialization/deserialization names, keyword args, etc.
 
 Most typical use-case is construct a custom type `T` from an `obj::AbstractDict`, but
 `constructfrom` is fully generic, so the inverse is also supported (turning any custom
-struct into an `AbstractDict`). For example, an external service may be providing JSON
+struct into an `AbstractDict`). A `CustomStruct` source is converted through its
+`lower` method. For example, an external service may be providing JSON
 data with an evolving schema; as opposed to trying a strict "typed parsing" like
 `JSON3.read(json, T)`, it may be preferrable to setup a local custom struct with just
 the desired properties and call `StructTypes.constructfrom(T, JSON3.read(json))`. This
@@ -1007,6 +1008,9 @@ constructfrom(::DictType, ::Type{T}, obj) where {T <: AbstractDict} =
 @inline constructfrom(::DictType, ::Type{T}, ::Type{K}, ::Type{V}, obj::S) where {T, K, V, S} =
     constructfrom(DictType(), T, K, V, StructType(S), obj)
 
+constructfrom(::DictType, ::Type{T}, ::Type{K}, ::Type{V}, ::CustomStruct, obj) where {T, K, V} =
+    constructfrom(DictType(), T, K, V, lower(obj))
+
 function constructfrom(::DictType, ::Type{T}, ::Type{K}, ::Type{V}, ::DictType, obj) where {T, K, V}
     d = Dict{K, V}()
     for (k, v) in keyvaluepairs(obj)
@@ -1068,6 +1072,9 @@ function constructfrom!(::Mutable, x::T, ::Union{Struct, Mutable}, obj) where {T
     return x
 end
 
+constructfrom!(::Mutable, x::T, ::CustomStruct, obj) where {T} =
+    constructfrom!(x, lower(obj))
+
 constructfrom(::Struct, ::Type{T}, obj::S) where {T, S} =
     constructfrom(Struct(), T, StructType(S), obj)
 
@@ -1102,6 +1109,9 @@ end
 constructfrom(::Struct, ::Type{T}, ::Union{Struct, Mutable}, obj) where {T} =
     construct(StructClosure(obj), T)
 
+constructfrom(::Struct, ::Type{T}, ::CustomStruct, obj) where {T} =
+    constructfrom(T, lower(obj))
+
 struct TupleClosure{T}
     obj::T
 end

test/runtests.jl

@@ -877,3 +877,42 @@ end
 @testset "struct type on complex" begin
     @test StructTypes.StructType(ComplexF64) == StructTypes.Struct()
 end
+
+struct CustomColumn{T, S}
+    values::Vector{T}
+end
+StructTypes.StructType(::Type{<:CustomColumn}) = StructTypes.CustomStruct()
+StructTypes.lowertype(::Type{<:CustomColumn}) = Dict{String, Any}
+StructTypes.lower(c::CustomColumn{T, S}) where {T, S} = Dict{String, Any}(
+    "type" => string(T), "storage" => string(S), "values" => c.values)
+
+mutable struct MutableColumn
+    type::String
+    values::Vector{Int}
+    MutableColumn() = new()
+end
+StructTypes.StructType(::Type{MutableColumn}) = StructTypes.Mutable()
+
+struct ImmutableColumn
+    type::String
+    values::Vector{Int}
+end
+StructTypes.StructType(::Type{ImmutableColumn}) = StructTypes.Struct()
+
+struct LowersToNamedTuple
+    x::NamedTuple
+end
+StructTypes.StructType(::Type{LowersToNamedTuple}) = StructTypes.CustomStruct()
+StructTypes.lower(x::LowersToNamedTuple) = x.x
+
+@testset "constructfrom with CustomStruct sources" begin
+    c = CustomColumn{Int, Nothing}([1, 2])
+    @test StructTypes.constructfrom(Dict{String, Any}, c) ==
+        Dict("type" => string(Int), "storage" => "Nothing", "values" => [1, 2])
+    @test StructTypes.constructfrom(Dict{Symbol, Any}, c)[:storage] == "Nothing"
+    w = LowersToNamedTuple((type="Int64", values=[1, 2]))
+    m = StructTypes.constructfrom(MutableColumn, w)
+    @test (m.type, m.values) == ("Int64", [1, 2])
+    s = StructTypes.constructfrom(ImmutableColumn, w)
+    @test (s.type, s.values) == ("Int64", [1, 2])
+end