# How to view which model the variable belongs to?

**URL:** <https://discourse.julialang.org/t/how-to-view-which-model-the-variable-belongs-to/67233>\
**Category:** Optimization (Mathematical)\
**Tags:** question, jump\
**Created:** [August 28, 2021, 4:08pm UTC](https://discourse.julialang.org/t/how-to-view-which-model-the-variable-belongs-to/67233 "2021-08-28T16:08:18Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![iiitr](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/iiitr/32/24586_2.png) [@iiitr](https://discourse.julialang.org/u/iiitr)\
**Post date:** [August 28, 2021, 4:08pm UTC](https://discourse.julialang.org/t/how-to-view-which-model-the-variable-belongs-to/67233/1 "2021-08-28T16:08:18Z")

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Hi, all

Here, I create two variables which belong to two different model, respectively, like:  
m1 = Model()  
m2 = Model()  
@variable(m1, x)  
@variable(m2, y)

How can I know x and y belong to which model?

I already know if i use the code following:  
x.model  
I may get the information about model m1.

but if i get a AffExpr, like z = x1 + x2, x1 and x2 belong to model m1  
z.model will push a error

Can some function can show the model information about AffExpr ?

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<div class="post-metadata">

**Author:** ![odow](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/odow/32/28685_2.png) [@odow](https://discourse.julialang.org/u/odow)\
**Post date:** [August 28, 2021, 9:43pm UTC](https://discourse.julialang.org/t/how-to-view-which-model-the-variable-belongs-to/67233/2 "2021-08-28T21:43:05Z")

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> How can I know x and y belong to which model?

For variables there is

```Julia
julia> owner_model(x) === m1
true

julia> owner_model(x) === m2
false

julia> owner_model(y) === m1
false

julia> owner_model(y) === m2
true

```

But not for `AffExpr`.

```Julia
julia> z = x + x
2 x

julia> owner_model(z)
ERROR: MethodError: no method matching owner_model(::AffExpr)
Closest candidates are:
  owner_model(::ConstraintRef) at /Users/oscar/.julia/packages/JuMP/b3hGi/src/constraints.jl:39
  owner_model(::AbstractVariableRef) at /Users/oscar/.julia/packages/JuMP/b3hGi/src/variables.jl:179
Stacktrace:
 [1] top-level scope
   @ REPL[20]:1

julia> owner_model(first(z.terms)[1])
A JuMP Model
Feasibility problem with:
Variable: 1
Model mode: AUTOMATIC
CachingOptimizer state: NO_OPTIMIZER
Solver name: No optimizer attached.
Names registered in the model: x

```

Why do you need this? You can probably structure your code to avoid needing this lookup.

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<div class="post-metadata">

**Author:** ![iiitr](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/iiitr/32/24586_2.png) [@iiitr](https://discourse.julialang.org/u/iiitr)\
**Post date:** [August 29, 2021, 1:17am UTC](https://discourse.julialang.org/t/how-to-view-which-model-the-variable-belongs-to/67233/3 "2021-08-29T01:17:39Z")

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For a very simple reason, I want to create a generalized constraint with absolute value, but I don’t want to write a few lines of constraints every time I create it. So I built a function, as shown below.

> function abs\_(model, x)  
> n, m = size(x)  
> str1 = randstring(5)  
> absx = @variable(model, [1:n, 1:m])  
> @constraints(model, begin  
> x .\<= absx  
> x .\>= -absx  
> end)  
> return absx

But I’m very lazy. I don’t even want to write the word “model”, so I’m looking for a way to identify the model to which the input variable or expression belongs

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<div class="post-metadata">

**Author:** ![odow](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/odow/32/28685_2.png) [@odow](https://discourse.julialang.org/u/odow)\
**Post date:** [August 29, 2021, 1:51am UTC](https://discourse.julialang.org/t/how-to-view-which-model-the-variable-belongs-to/67233/4 "2021-08-29T01:51:37Z")

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I’ve opend a feature request: [https://github.com/jump-dev/JuMP.jl/issues/2678](https://github.com/jump-dev/JuMP.jl/issues/2678)

For now, you could do something like this:

```julia
using JuMP
_owner_model(x::VariableRef) = owner_model(x)
_owner_model(x::AffExpr) = owner_model(first(x.terms)[1])
function my_abs(x::Union{VariableRef,AffExpr})
    model = _owner_model(x)
    abs_x = @variable(model)
    @constraint(model, x <= abs_x)
    @constraint(model, x >= abs_x)
    return abs_x
end

model = Model()
@variable(model, x[1:2, 1:2])
y = my_abs.(x)
z = my_abs.(x .+ x')

```

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<div class="post-metadata">

**Author:** ![iiitr](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/iiitr/32/24586_2.png) [@iiitr](https://discourse.julialang.org/u/iiitr)\
**Post date:** [August 29, 2021, 1:59am UTC](https://discourse.julialang.org/t/how-to-view-which-model-the-variable-belongs-to/67233/5 "2021-08-29T01:59:17Z")

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Oh, thank you very much!  
At present, the best solution seems to be to add a new method whenever I encounter an error code.