# Way to migrate jld2 data to have new fields without corruption?

**URL:** <https://discourse.julialang.org/t/way-to-migrate-jld2-data-to-have-new-fields-without-corruption/15309>\
**Category:** General Usage\
**Tags:** question\
**Created:** [September 21, 2018, 7:29pm UTC](https://discourse.julialang.org/t/way-to-migrate-jld2-data-to-have-new-fields-without-corruption/15309 "2018-09-21T19:29:57Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![djsegal](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/djsegal/32/13752_2.png) [@djsegal](https://discourse.julialang.org/u/djsegal)\
**Post date:** [September 21, 2018, 7:29pm UTC](https://discourse.julialang.org/t/way-to-migrate-jld2-data-to-have-new-fields-without-corruption/15309/1 "2018-09-21T19:29:57Z")

</div>

Let’s say you have a struct, `Foo`:

```julia
struct Foo
  fizz::Int
end

```

Then you make a slew of Foo’s and save them to a file using JLD2.jl

* * *

Now, your boss tells you to add a `buzz` flag:

```julia
struct Foo
  fizz::Int 
  buzz::Bool
end

```

On booting up a jupyter notebook, you now find your `N` hours of Foo’s are corrupted:

```julia
WARNING: saved type Bar.Foo is missing field buzz in workspace type; reconstructing
Error encountered while loading "/Users/picard/Desktop/notebook_folder/foo.jld2".
Fatal error:

```

```julia
MethodError: Cannot `convert` an object of type JLD2.ReconstructedTypes.##Bar.Foo#673 to an object of type Bar.AbstractFoo
This may have arisen from a call to the constructor Bar.AbstractFoo(...),
since type constructors fall back to convert methods.

```

* * *

Is there a way you can work around this issue?

// it seems like migrations are a fundamental building block of storing data

---

<div class="post-metadata">

**Author:** ![kristoffer.carlsson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kristoffer.carlsson/32/22_2.png) [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)\
**Post date:** [September 21, 2018, 7:37pm UTC](https://discourse.julialang.org/t/way-to-migrate-jld2-data-to-have-new-fields-without-corruption/15309/2 "2018-09-21T19:37:43Z")

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Is that really a full example? What is `Bar.AbstractFoo`?

```julia
julia> struct Foo
         fizz::Int
       end

julia> a = [Foo(1), Foo(2)]
2-element Array{Foo,1}:
 Foo(1)
 Foo(2)

julia> using JLD2
[Info: Recompiling stale cache file /Users/kristoffer/.julia/compiled/v1.1/JLD2/O1EyT.ji for JLD2 [033835bb-8acc-5ee8-8aae-3f567f8a3819]

julia> @save "foo.jld" a

➜ ~/julia/julia -q # restart Julia

julia> struct Foo
         fizz::Int
         bar::Bool
       end

julia> using JLD2

julia> @load "foo.jld" a
┌ Warning: saved type Foo is missing field bar in workspace type; reconstructing
└ @ JLD2 ~/.julia/packages/JLD2/KjBIK/src/data.jl:402
1-element Array{Symbol,1}:
 :a

julia> a
2-element Array{getfield(JLD2.ReconstructedTypes, Symbol("##Foo#360")),1}:
 getfield(JLD2.ReconstructedTypes, Symbol("##Foo#360"))(1)
 getfield(JLD2.ReconstructedTypes, Symbol("##Foo#360"))(2)

julia> a[1]
getfield(JLD2.ReconstructedTypes, Symbol("##Foo#360"))(1)

julia> a[1].fizz
1

```

---

<div class="post-metadata">

**Author:** ![djsegal](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/djsegal/32/13752_2.png) [@djsegal](https://discourse.julialang.org/u/djsegal)\
**Post date:** [September 21, 2018, 7:42pm UTC](https://discourse.julialang.org/t/way-to-migrate-jld2-data-to-have-new-fields-without-corruption/15309/3 "2018-09-21T19:42:21Z")

</div>

First, this is the actual struct:

- [https://github.com/djsegal/Fussy.jl/blob/ba8918f01e2b0ffa548ff9382c4aa9af1bce9f17/src/structs/reactor.jl#L116](https://github.com/djsegal/Fussy.jl/blob/ba8918f01e2b0ffa548ff9382c4aa9af1bce9f17/src/structs/reactor.jl#L116)

// with `Foo` mapping to `Reactor` and `buzz` mapping to `closure`

* * *

And `AbstractFoo` is something I use so functions can be built before I define the `Foo` type (i.e. for load order).

See:

> <https://github.com/djsegal/Fussy.jl/blob/master/src/abstracts.jl>

---

<div class="post-metadata">

**Author:** ![djsegal](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/djsegal/32/13752_2.png) [@djsegal](https://discourse.julialang.org/u/djsegal)\
**Post date:** [September 22, 2018, 8:01pm UTC](https://discourse.julialang.org/t/way-to-migrate-jld2-data-to-have-new-fields-without-corruption/15309/4 "2018-09-22T20:01:43Z")

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I just decided to delete the new field and never change the struct ever again.

Bada bing. Bada boom. Problem solved.