`complete` swaps order of metadata in `ReactionSystem`s

[TorkelE]

For some reason, in some cases where you do complete on a ReactionSystem, that can swap around the order on metadata. Since metadata is not overwritten, but just added to the metadata dict, that means that what is retrieved from getmetadata can change.

In principle, this just involves MTK functions, so everything should transfer to ReactionSystem. However, since things work fine for System, something is obviously going on in Catalyst.

using Catalyst, ModelingToolkitBase
t = default_t()
D = default_time_deriv()
# Create model.
@species X(t)
@parameters d
@named rs2 = ReactionSystem([Reaction(d, [X], nothing)], t; metadata = [MiscSystemData => π])
rs2 = ModelingToolkitBase.setmetadata(rs2, MiscSystemData, ones(2, 3))
ModelingToolkitBase.getmetadata(complete(rs2), MiscSystemData, nothing) # π = 3.1415926535897... (incorrect)

@species X(t)
@parameters d
@named sys2 = System([Differential(t)(X) ~ -d*X], t; metadata = [MiscSystemData => π])
sys2 = ModelingToolkitBase.setmetadata(sys2, MiscSystemData, ones(2, 3))
ModelingToolkitBase.getmetadata(complete(sys2), MiscSystemData, nothing) # 2×3 Matrix{Float64}: (correct)

And the actual metadata dicts are:

julia> ModelingToolkitBase.get_metadata((rs2))
Base.ImmutableDict{DataType, Any} with 3 entries:
  MiscSystemData  => [1.0 1.0 1.0; 1.0 1.0 1.0]
  MutableCacheKey => Dict{DataType, Any}()
  MiscSystemData  => π

julia> ModelingToolkitBase.get_metadata(complete(rs2))
Base.ImmutableDict{DataType, Any} with 3 entries:
  MiscSystemData  => π
  MutableCacheKey => Dict{DataType, Any}()
  MiscSystemData  => [1.0 1.0 1.0; 1.0 1.0 1.0]

julia> ModelingToolkitBase.get_metadata((sys2))
Base.ImmutableDict{DataType, Any} with 3 entries:
  MiscSystemData  => [1.0 1.0 1.0; 1.0 1.0 1.0]
  MutableCacheKey => Dict{DataType, Any}()
  MiscSystemData  => π

julia> ModelingToolkitBase.get_metadata(complete(sys2))
Base.ImmutableDict{DataType, Any} with 3 entries:
  MiscSystemData  => [1.0 1.0 1.0; 1.0 1.0 1.0]
  MutableCacheKey => Dict{DataType, Any}()
  MiscSystemData  => π

This is only relevant on the current branch for Catalyst v16 (but will likely remain after that merges)

[TorkelE]

@AayushSabharwal Once everything else is sorted out, this is one I might need your help with.

[isaacsas]

Why does the ordering matter? Seems like any testing of dicts should just test set equality.

[TorkelE]

I don't really know, but when you do

rs2 = ModelingToolkitBase.setmetadata(rs2, MiscSystemData, ones(2, 3))

the old MiscSystemData entry is actually not removed, but a new one added (this is intended behaviour: https://github.com/SciML/ModelingToolkit.jl/issues/4169). What I think the happens is that ModelingToolkitBase.getmetadata has some way of determining which MiscSystemData entry should be fetched. However, when complete flips the order, ModelingToolkitBase.getmetadata's methodology for determening which one to get fails, and it fetches the wrong one.

I don't really know what happens internally though, so that is just a theory.