incorrect units in observed equations

[aml5600]

Describe the bug 🐞

After calling structural_simplify, the returned system's observed equations have missing/incorrect units if the original model has a) units and b) @constants

I imagine this is another case of missing a subs_constants somewhere.

I plan to make a PR to address this, but any hints of where to look would be appreciated.

Expected behavior

The units of the lhs and rhs of observed equations should match

Minimal Reproducible Example 👇

using ModelingToolkit
using DynamicQuantities
using Test
using ModelingToolkit: t, D

@mtkmodel TestSys begin
    @constants begin
        to_m = 1, [unit = u"m"]
    end
    @parameters begin
        v, [unit = u"m/s"]
    end
    @variables begin
        L(t), [unit = u"m"]
        L_out(t), [unit = u"1"]
    end
    @equations begin
        D(L) ~ v
        (L_out * to_m) ~ L
    end
end

@named sys = TestSys()
sys_simple = structural_simplify(sys)
obs = only(observed(sys_simple))
@test ModelingToolkit.get_unit(obs.lhs) == ModelingToolkit.get_unit(obs.rhs) 

Error & Stacktrace ⚠️

Test Failed at REPL[73]:1
  Expression: ModelingToolkit.get_unit(obs.lhs) == ModelingToolkit.get_unit(obs.rhs)
   Evaluated: 1.0  == 1.0 m

ERROR: There was an error during testing

Environment (please complete the following information):

• Output of using Pkg; Pkg.status()

julia> using Pkg; Pkg.status()
Project ModelingToolkit v9.33.1
Status `~/workspace/ModelingToolkit.jl/Project.toml`
  [1520ce14] AbstractTrees v0.4.5
  [4fba245c] ArrayInterface v7.15.0
  [8e7c35d0] BlockArrays v1.1.0
  [861a8166] Combinatorics v1.0.2
  [34da2185] Compat v4.16.0
  [187b0558] ConstructionBase v1.5.7
  [864edb3b] DataStructures v0.18.20
  [2b5f629d] DiffEqBase v6.154.0
  [459566f4] DiffEqCallbacks v3.8.0
  [77a26b50] DiffEqNoiseProcess v5.23.0
  [b552c78f] DiffRules v1.15.1
  [31c24e10] Distributions v0.25.110
  [ffbed154] DocStringExtensions v0.9.3
  [5b8099bc] DomainSets v0.7.14
⌅ [06fc5a27] DynamicQuantities v0.13.2
  [e2ba6199] ExprTools v0.1.10
⌅ [6b7a57c9] Expronicon v0.8.5
  [64ca27bc] FindFirstFunctions v1.3.0
  [f6369f11] ForwardDiff v0.10.36
  [77dc65aa] FunctionWrappersWrappers v0.1.3
  [86223c79] Graphs v1.11.2
  [98e50ef6] JuliaFormatter v1.0.59
  [ccbc3e58] JumpProcesses v9.13.3
  [23fbe1c1] Latexify v0.16.5
  [d8e11817] MLStyle v0.4.17
  [77ba4419] NaNMath v1.0.2
  [8913a72c] NonlinearSolve v3.14.0
  [bac558e1] OrderedCollections v1.6.3
  [aea7be01] PrecompileTools v1.2.1
  [731186ca] RecursiveArrayTools v3.27.0
  [189a3867] Reexport v1.2.2
  [7e49a35a] RuntimeGeneratedFunctions v0.5.13
  [0bca4576] SciMLBase v2.50.1
  [53ae85a6] SciMLStructures v1.4.2
  [efcf1570] Setfield v1.1.1
  [727e6d20] SimpleNonlinearSolve v1.12.0
  [276daf66] SpecialFunctions v2.4.0
  [90137ffa] StaticArrays v1.9.7
  [2efcf032] SymbolicIndexingInterface v0.3.28
  [d1185830] SymbolicUtils v3.4.0
  [0c5d862f] Symbolics v6.3.0
  [5c2747f8] URIs v1.5.1
  [3a884ed6] UnPack v1.0.2
  [1986cc42] Unitful v1.21.0
  [8ba89e20] Distributed
  [b77e0a4c] InteractiveUtils
  [8f399da3] Libdl
  [37e2e46d] LinearAlgebra
  [9e88b42a] Serialization
  [2f01184e] SparseArrays v1.10.0

• Output of using Pkg; Pkg.status(; mode = PKGMODE_MANIFEST)

julia> using Pkg; Pkg.status(; mode = PKGMODE_MANIFEST)
Project ModelingToolkit v9.33.1
Status `~/workspace/ModelingToolkit.jl/Manifest.toml`
  [47edcb42] ADTypes v1.7.1
  [1520ce14] AbstractTrees v0.4.5
  [7d9f7c33] Accessors v0.1.37
  [79e6a3ab] Adapt v4.0.4
  [66dad0bd] AliasTables v1.1.3
  [ec485272] ArnoldiMethod v0.4.0
  [4fba245c] ArrayInterface v7.15.0
  [4c555306] ArrayLayouts v1.10.3
  [e2ed5e7c] Bijections v0.1.7
  [62783981] BitTwiddlingConvenienceFunctions v0.1.6
  [8e7c35d0] BlockArrays v1.1.0
  [2a0fbf3d] CPUSummary v0.2.6
  [00ebfdb7] CSTParser v3.4.3
  [d360d2e6] ChainRulesCore v1.24.0
  [fb6a15b2] CloseOpenIntervals v0.1.13
  [861a8166] Combinatorics v1.0.2
  [a80b9123] CommonMark v0.8.12
  [38540f10] CommonSolve v0.2.4
  [bbf7d656] CommonSubexpressions v0.3.1
  [f70d9fcc] CommonWorldInvalidations v1.0.0
  [34da2185] Compat v4.16.0
  [b152e2b5] CompositeTypes v0.1.4
  [a33af91c] CompositionsBase v0.1.2
  [2569d6c7] ConcreteStructs v0.2.3
  [187b0558] ConstructionBase v1.5.7
  [adafc99b] CpuId v0.3.1
  [a8cc5b0e] Crayons v4.1.1
  [9a962f9c] DataAPI v1.16.0
  [864edb3b] DataStructures v0.18.20
  [e2d170a0] DataValueInterfaces v1.0.0
  [2b5f629d] DiffEqBase v6.154.0
  [459566f4] DiffEqCallbacks v3.8.0
  [77a26b50] DiffEqNoiseProcess v5.23.0
  [163ba53b] DiffResults v1.1.0
  [b552c78f] DiffRules v1.15.1
  [a0c0ee7d] DifferentiationInterface v0.5.14
  [31c24e10] Distributions v0.25.110
  [ffbed154] DocStringExtensions v0.9.3
  [5b8099bc] DomainSets v0.7.14
  [7c1d4256] DynamicPolynomials v0.6.0
⌅ [06fc5a27] DynamicQuantities v0.13.2
  [4e289a0a] EnumX v1.0.4
  [f151be2c] EnzymeCore v0.7.8
  [e2ba6199] ExprTools v0.1.10
⌅ [6b7a57c9] Expronicon v0.8.5
  [7034ab61] FastBroadcast v0.3.5
  [9aa1b823] FastClosures v0.3.2
  [29a986be] FastLapackInterface v2.0.4
  [1a297f60] FillArrays v1.12.0
  [64ca27bc] FindFirstFunctions v1.3.0
  [6a86dc24] FiniteDiff v2.24.0
  [1fa38f19] Format v1.3.7
  [f6369f11] ForwardDiff v0.10.36
  [069b7b12] FunctionWrappers v1.1.3
  [77dc65aa] FunctionWrappersWrappers v0.1.3
  [d9f16b24] Functors v0.4.12
  [46192b85] GPUArraysCore v0.1.6
  [c27321d9] Glob v1.3.1
  [86223c79] Graphs v1.11.2
  [3e5b6fbb] HostCPUFeatures v0.1.17
  [34004b35] HypergeometricFunctions v0.3.24
  [615f187c] IfElse v0.1.1
  [d25df0c9] Inflate v0.1.5
  [18e54dd8] IntegerMathUtils v0.1.2
  [8197267c] IntervalSets v0.7.10
  [3587e190] InverseFunctions v0.1.16
  [92d709cd] IrrationalConstants v0.2.2
  [82899510] IteratorInterfaceExtensions v1.0.0
  [692b3bcd] JLLWrappers v1.5.0
  [682c06a0] JSON v0.21.4
  [98e50ef6] JuliaFormatter v1.0.59
  [ccbc3e58] JumpProcesses v9.13.3
  [ef3ab10e] KLU v0.6.0
  [ba0b0d4f] Krylov v0.9.6
  [b964fa9f] LaTeXStrings v1.3.1
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  [10f19ff3] LayoutPointers v0.1.17
  [5078a376] LazyArrays v2.2.0
  [d3d80556] LineSearches v7.3.0
  [7ed4a6bd] LinearSolve v2.33.0
  [2ab3a3ac] LogExpFunctions v0.3.28
  [bdcacae8] LoopVectorization v0.12.171
  [d8e11817] MLStyle v0.4.17
  [1914dd2f] MacroTools v0.5.13
  [d125e4d3] ManualMemory v0.1.8
  [bb5d69b7] MaybeInplace v0.1.3
  [e1d29d7a] Missings v1.2.0
  [46d2c3a1] MuladdMacro v0.2.4
  [102ac46a] MultivariatePolynomials v0.5.6
  [d8a4904e] MutableArithmetics v1.4.6
  [d41bc354] NLSolversBase v7.8.3
  [77ba4419] NaNMath v1.0.2
  [8913a72c] NonlinearSolve v3.14.0
  [6fe1bfb0] OffsetArrays v1.14.1
  [429524aa] Optim v1.9.4
  [bac558e1] OrderedCollections v1.6.3
  [90014a1f] PDMats v0.11.31
  [65ce6f38] PackageExtensionCompat v1.0.2
  [d96e819e] Parameters v0.12.3
  [69de0a69] Parsers v2.8.1
  [e409e4f3] PoissonRandom v0.4.4
  [f517fe37] Polyester v0.7.16
  [1d0040c9] PolyesterWeave v0.2.2
  [85a6dd25] PositiveFactorizations v0.2.4
  [d236fae5] PreallocationTools v0.4.24
  [aea7be01] PrecompileTools v1.2.1
  [21216c6a] Preferences v1.4.3
  [27ebfcd6] Primes v0.5.6
  [43287f4e] PtrArrays v1.2.0
  [1fd47b50] QuadGK v2.10.1
  [74087812] Random123 v1.7.0
  [e6cf234a] RandomNumbers v1.6.0
  [3cdcf5f2] RecipesBase v1.3.4
  [731186ca] RecursiveArrayTools v3.27.0
  [f2c3362d] RecursiveFactorization v0.2.23
  [189a3867] Reexport v1.2.2
  [ae029012] Requires v1.3.0
  [ae5879a3] ResettableStacks v1.1.1
  [79098fc4] Rmath v0.7.1
  [7e49a35a] RuntimeGeneratedFunctions v0.5.13
  [94e857df] SIMDTypes v0.1.0
  [476501e8] SLEEFPirates v0.6.43
  [0bca4576] SciMLBase v2.50.1
  [c0aeaf25] SciMLOperators v0.3.9
  [53ae85a6] SciMLStructures v1.4.2
  [efcf1570] Setfield v1.1.1
  [727e6d20] SimpleNonlinearSolve v1.12.0
  [699a6c99] SimpleTraits v0.9.4
  [a2af1166] SortingAlgorithms v1.2.1
  [47a9eef4] SparseDiffTools v2.20.0
  [0a514795] SparseMatrixColorings v0.4.0
  [e56a9233] Sparspak v0.3.9
  [276daf66] SpecialFunctions v2.4.0
  [aedffcd0] Static v1.1.1
  [0d7ed370] StaticArrayInterface v1.8.0
  [90137ffa] StaticArrays v1.9.7
  [1e83bf80] StaticArraysCore v1.4.3
  [82ae8749] StatsAPI v1.7.0
  [2913bbd2] StatsBase v0.34.3
  [4c63d2b9] StatsFuns v1.3.1
  [7792a7ef] StrideArraysCore v0.5.7
  [2efcf032] SymbolicIndexingInterface v0.3.28
  [19f23fe9] SymbolicLimits v0.2.2
  [d1185830] SymbolicUtils v3.4.0
  [0c5d862f] Symbolics v6.3.0
  [3783bdb8] TableTraits v1.0.1
  [bd369af6] Tables v1.12.0
  [8ea1fca8] TermInterface v2.0.0
  [8290d209] ThreadingUtilities v0.5.2
  [a759f4b9] TimerOutputs v0.5.24
  [0796e94c] Tokenize v0.5.29
  [d5829a12] TriangularSolve v0.2.1
  [410a4b4d] Tricks v0.1.9
  [781d530d] TruncatedStacktraces v1.4.0
  [5c2747f8] URIs v1.5.1
  [3a884ed6] UnPack v1.0.2
  [1986cc42] Unitful v1.21.0
  [a7c27f48] Unityper v0.1.6
  [3d5dd08c] VectorizationBase v0.21.70
  [19fa3120] VertexSafeGraphs v0.2.0
  [1d5cc7b8] IntelOpenMP_jll v2024.2.0+0
  [856f044c] MKL_jll v2024.2.0+0
  [efe28fd5] OpenSpecFun_jll v0.5.5+0
⌅ [f50d1b31] Rmath_jll v0.4.3+0
  [1317d2d5] oneTBB_jll v2021.12.0+0
  [0dad84c5] ArgTools v1.1.1
  [56f22d72] Artifacts
  [2a0f44e3] Base64
  [ade2ca70] Dates
  [8ba89e20] Distributed
  [f43a241f] Downloads v1.6.0
  [7b1f6079] FileWatching
  [9fa8497b] Future
  [b77e0a4c] InteractiveUtils
  [4af54fe1] LazyArtifacts
  [b27032c2] LibCURL v0.6.4
  [76f85450] LibGit2
  [8f399da3] Libdl
  [37e2e46d] LinearAlgebra
  [56ddb016] Logging
  [d6f4376e] Markdown
  [a63ad114] Mmap
  [ca575930] NetworkOptions v1.2.0
  [44cfe95a] Pkg v1.10.0
  [de0858da] Printf
  [3fa0cd96] REPL
  [9a3f8284] Random
  [ea8e919c] SHA v0.7.0
  [9e88b42a] Serialization
  [1a1011a3] SharedArrays
  [6462fe0b] Sockets
  [2f01184e] SparseArrays v1.10.0
  [10745b16] Statistics v1.10.0
  [4607b0f0] SuiteSparse
  [fa267f1f] TOML v1.0.3
  [a4e569a6] Tar v1.10.0
  [8dfed614] Test
  [cf7118a7] UUIDs
  [4ec0a83e] Unicode
  [e66e0078] CompilerSupportLibraries_jll v1.1.1+0
  [deac9b47] LibCURL_jll v8.4.0+0
  [e37daf67] LibGit2_jll v1.6.4+0
  [29816b5a] LibSSH2_jll v1.11.0+1
  [c8ffd9c3] MbedTLS_jll v2.28.2+1
  [14a3606d] MozillaCACerts_jll v2023.1.10
  [4536629a] OpenBLAS_jll v0.3.23+4
  [05823500] OpenLibm_jll v0.8.1+2
  [bea87d4a] SuiteSparse_jll v7.2.1+1
  [83775a58] Zlib_jll v1.2.13+1
  [8e850b90] libblastrampoline_jll v5.8.0+1
  [8e850ede] nghttp2_jll v1.52.0+1
  [3f19e933] p7zip_jll v17.4.0+2

• Output of versioninfo()

julia> versioninfo()
Julia Version 1.10.4
Commit 48d4fd48430 (2024-06-04 10:41 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: macOS (arm64-apple-darwin22.4.0)
  CPU: 8 × Apple M1 Pro
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-15.0.7 (ORCJIT, apple-m1)
Threads: 8 default, 0 interactive, 4 GC (on 6 virtual cores)
Environment:
  JULIA_EDITOR = code
  JULIA_NUM_THREADS = 8

Additional context

Add any other context about the problem here.

[aml5600]

I've spent the morning trying to dig around and find the source of my unit issues...from what I can tell, my units are getting dropped in the alias elimination part of structural_simplify, e.g. around here.

I have tried:

• modifying fold_constants to not sub constants if they have units, this broke structural_simplify
• modifying my constants default value to have the same units as it's metadata, this breaks stuff a few lines above the link above in linear_expansion
• modifying getdefault to add units during substitution.

None of these have worked unfortunately. Any hints? my take away is that it seems like constants should be forced to be unitless? Which is unfortunate as using them for unit conversions (specifically with RealInput and RealOutput) is quite convenient.

[aml5600]

Would it be feasible to add optional units to RealInput and RealOutput, essentially making the constants-as-unit-helpers redundant?

[ChrisRackauckas]

Yes that would be feasible.