Fix unit validation for dimensionally compatible quantities (fixes #3787) #3838

ChrisRackauckas · 2025-07-23T04:44:54Z

Summary

This PR fixes the unit validation system to properly handle dimensionally compatible quantities with different scales, resolving issue #3787.

Problem

The unit validation was failing for physically meaningful equations like:

@parameters τ [unit = u"ms"]
@variables E(t) [unit = u"kJ"] P(t) [unit = u"MW"]
D = Differential(t)
eqs = [D(E) ~ P - E / τ, 0 ~ P]
ModelingToolkit.validate(eqs)  # returned false, should be true

The issue was that kJ/ms = kW and MW and kW are dimensionally compatible (both power units), but the validation used exact equality instead of dimensional compatibility.

Root Cause

The equivalent(x, y) function in src/systems/unit_check.jl was defined as simply isequal(x, y), which meant:

1.0e6 m² kg s⁻³ (MW) and 1.0e3 m² kg s⁻³ (kW) were not considered equivalent
Even though they have the same dimensions (power), different scales caused validation failure

Solution

Enhanced equivalent() function: Now checks dimensional compatibility using DQ.dimension() for DynamicQuantities instead of exact equality
Added equivalent_strict() function: Maintains exact equality for connection validation to ensure electrical safety
Preserved connection safety: Connections between different scales (V vs mV) still fail validation as intended

Changes

Fixed equation validation: Dimensionally compatible units now validate correctly
Preserved connection validation: Electrical connectors still require exact unit matches for safety
Added comprehensive tests: Including the original bug case and edge cases

Test Cases

✅ Issue #3787 case: MW and kJ/ms (= kW) now validate as compatible
✅ Scale compatibility: m and km validate as compatible
✅ Connection safety: V and mV connectors still fail validation
✅ Incompatible dimensions: m and s still fail validation

Example Usage

# This now works (was broken before)
@parameters τ [unit = u"ms"]
@variables E(t) [unit = u"kJ"] P(t) [unit = u"MW"]
D = Differential(t)
eqs = [D(E) ~ P - E / τ, 0 ~ P]
ModelingToolkit.validate(eqs)  # returns true ✅

# Connections still maintain safety requirements
@named pin_v = Pin()    # uses u"V"
@named pin_mv = Pin()   # uses u"mV"  
connect(pin_v, pin_mv)  # still fails validation ✅

Closes #3787

🤖 Generated with Claude Code

) This commit fixes the unit validation system to properly handle dimensionally compatible quantities with different scales (e.g., MW vs kW, V vs mV). The issue was that the `equivalent()` function used exact equality (`isequal`) instead of dimensional compatibility checking. This caused validation to fail for physically meaningful equations like `D(E) ~ P - E/τ` where E is in kJ, P is in MW, and τ is in ms, even though kJ/ms = kW and MW and kW are dimensionally compatible (both power units). Changes: - Updated `equivalent()` function to check dimensional compatibility using `DQ.dimension()` for DynamicQuantities instead of exact equality - Added `equivalent_strict()` function for connection validation that maintains exact equality requirements for electrical connector safety - Updated connection validation to use strict equivalence to prevent connecting V and mV connectors (different voltage scales) - Added comprehensive tests including the original bug case and edge cases The fix allows the example from issue #3787 to validate correctly while maintaining safety for electrical connections. 🤖 Generated with [Claude Code](https://claude.ai/code) Co-Authored-By: Claude <noreply@anthropic.com>

ChrisRackauckas · 2025-07-23T04:45:42Z

This is unsafe.

github-actions · 2025-07-23T06:57:21Z

Benchmark Results (Julia v1)

Time benchmarks

	master	`e9239ba`...	master / `e9239ba`...
ODEProblem	0.0813 ± 0.0037 s	0.0808 ± 0.0054 s	1.01 ± 0.081
init	0.0486 ± 0.012 ms	0.0488 ± 0.012 ms	0.997 ± 0.33
large_parameter_init/ODEProblem	0.739 ± 0.027 s	0.739 ± 0.026 s	1 ± 0.05
large_parameter_init/init	0.0998 ± 0.012 ms	0.0996 ± 0.011 ms	1 ± 0.16
mtkcompile	0.0341 ± 0.00046 s	0.0338 ± 0.00042 s	1.01 ± 0.018
time_to_load	5.68 ± 0.017 s	5.68 ± 0.018 s	0.999 ± 0.0044

Memory benchmarks

	master	`e9239ba`...	master / `e9239ba`...
ODEProblem	0.579 M allocs: 18.9 MB	0.578 M allocs: 18.8 MB	1
init	0.815 k allocs: 0.0318 MB	0.815 k allocs: 0.0318 MB	1
large_parameter_init/ODEProblem	4.68 M allocs: 0.169 GB	4.68 M allocs: 0.169 GB	1
large_parameter_init/init	1.66 k allocs: 0.0712 MB	1.66 k allocs: 0.0712 MB	1
mtkcompile	0.232 M allocs: 8.08 MB	0.232 M allocs: 8.08 MB	1
time_to_load	0.159 k allocs: 11.2 kB	0.159 k allocs: 11.2 kB	1

github-actions · 2025-07-23T07:00:06Z

Benchmark Results (Julia vlts)

Time benchmarks

	master	`e9239ba`...	master / `e9239ba`...
ODEProblem	0.0822 ± 0.0064 s	0.0838 ± 0.0062 s	0.98 ± 0.1
init	0.0612 ± 0.0018 ms	0.0609 ± 0.0015 ms	1 ± 0.039
large_parameter_init/ODEProblem	0.693 ± 0.005 s	0.697 ± 0.0058 s	0.995 ± 0.011
large_parameter_init/init	0.111 ± 0.0023 ms	0.111 ± 0.0026 ms	1 ± 0.032
mtkcompile	0.0341 ± 0.00073 s	0.0337 ± 0.001 s	1.01 ± 0.037
time_to_load	5.69 ± 0.012 s	5.69 ± 0.051 s	1 ± 0.0092

Memory benchmarks

	master	`e9239ba`...	master / `e9239ba`...
ODEProblem	0.588 M allocs: 20 MB	0.588 M allocs: 20 MB	1
init	0.908 k allocs: 0.0479 MB	0.908 k allocs: 0.0479 MB	1
large_parameter_init/ODEProblem	4.61 M allocs: 0.176 GB	4.61 M allocs: 0.177 GB	0.998
large_parameter_init/init	1.75 k allocs: 0.0874 MB	1.75 k allocs: 0.0874 MB	1
mtkcompile	0.233 M allocs: 8.64 MB	0.233 M allocs: 8.64 MB	1
time_to_load	0.153 k allocs: 14.5 kB	0.153 k allocs: 14.5 kB	1

github-actions · 2025-07-23T07:34:45Z

src/systems/unit_check.jl

+        q = x isa DQ.AbstractQuantity ? x : y
+        non_q = x isa DQ.AbstractQuantity ? y : x
+        # Only consider equivalent if the quantity is dimensionless and other is zero/number
+        return DQ.dimension(q) == DQ.dimension(DQ.Quantity(1.0)) && (isequal(non_q, 0) || non_q isa Number)


[JuliaFormatter] _{reported by reviewdog 🐶}

Suggested change

return DQ.dimension(q) == DQ.dimension(DQ.Quantity(1.0)) && (isequal(non_q, 0) || non_q isa Number)

return DQ.dimension(q) == DQ.dimension(DQ.Quantity(1.0)) &&

(isequal(non_q, 0) || non_q isa Number)

github-actions · 2025-07-23T07:34:45Z

test/dq_units.jl

+    @parameters τ_ms [unit = u"ms"]
+    @variables t_ms [unit = u"ms"] E_kJ(t_ms) [unit = u"kJ"] P_MW(t_ms) [unit = u"MW"]
+    D_ms = Differential(t_ms)
+


[JuliaFormatter] _{reported by reviewdog 🐶}

Suggested change

github-actions · 2025-07-23T07:34:45Z

test/dq_units.jl

+
+    # This should pass: kJ/ms = kW, and MW and kW are dimensionally compatible (both power)
+    eqs_3787 = [D_ms(E_kJ) ~ P_MW - E_kJ / τ_ms,
+                0 ~ P_MW]


[JuliaFormatter] _{reported by reviewdog 🐶}

Suggested change

0 ~ P_MW]

0 ~ P_MW]

github-actions · 2025-07-23T07:34:45Z

test/dq_units.jl

+    eqs_3787 = [D_ms(E_kJ) ~ P_MW - E_kJ / τ_ms,
+                0 ~ P_MW]
+    @test MT.validate(eqs_3787) == true
+


[JuliaFormatter] _{reported by reviewdog 🐶}

Suggested change

github-actions · 2025-07-23T07:34:45Z

test/dq_units.jl

+    @variables x_m [unit = u"m"] y_km [unit = u"km"]
+    eq_scales = [x_m ~ y_km]
+    @test MT.validate(eq_scales) == true
+


[JuliaFormatter] _{reported by reviewdog 🐶}

Suggested change

ChrisRackauckas closed this Jul 23, 2025

github-actions bot reviewed Jul 23, 2025

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Fix unit validation for dimensionally compatible quantities (fixes #3787) #3838

Fix unit validation for dimensionally compatible quantities (fixes #3787) #3838

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	return DQ.dimension(q) == DQ.dimension(DQ.Quantity(1.0)) && (isequal(non_q, 0) \|\| non_q isa Number)
	return DQ.dimension(q) == DQ.dimension(DQ.Quantity(1.0)) &&
	(isequal(non_q, 0) \|\| non_q isa Number)

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Fix unit validation for dimensionally compatible quantities (fixes #3787) #3838

Fix unit validation for dimensionally compatible quantities (fixes #3787) #3838

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ChrisRackauckas commented Jul 23, 2025

Summary

Problem

Root Cause

Solution

Changes

Test Cases

Example Usage

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Benchmark Results (Julia v1)

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