# Store dataframes in structure and storing multiple database connection in connection pool

**URL:** <https://discourse.julialang.org/t/store-dataframes-in-structure-and-storing-multiple-database-connection-in-connection-pool/102467>\
**Category:** General Usage\
**Tags:** struct, julia\
**Created:** [August 4, 2023, 1:41am UTC](https://discourse.julialang.org/t/store-dataframes-in-structure-and-storing-multiple-database-connection-in-connection-pool/102467 "2023-08-04T01:41:25Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Sandy45](https://avatars.discourse-cdn.com/v4/letter/s/3ec8ea/32.png) [@Sandy45](https://discourse.julialang.org/u/Sandy45)\
**Post date:** [August 4, 2023, 1:41am UTC](https://discourse.julialang.org/t/store-dataframes-in-structure-and-storing-multiple-database-connection-in-connection-pool/102467/1 "2023-08-04T01:41:26Z")

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Q1 - I am working with multiple dataframes and trying to create structure with dataframes in it so that they can be accessed easily. I’ve created struct like below and when I try to assign the data to it I got an error “setfield! fields of Types should not be changed” despite of defining mutable struct. I’ve also tried using macro @set from Setfield package.

```julia
mutable struct all_dataframes1
       map_df::DataFrame
       code_df::DataFrame
       end

s = @set all_dataframes.map_df = df

ERROR: UndefRefError: access to undefined reference
Stacktrace:
  [1] getproperty
    @ .\Base.jl:37 [inlined]
  [2] _broadcast_getindex_evalf

Q2 - I am trying to store multiple database connections (eg. MYSQL and PostgreSQL) in connection pool so that I can access at any point in my code. I've seen there is connectionpool.jl that has been created for Redis but haven't found in packages registry.
```

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**Author:** ![algunion](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/algunion/32/51630_2.png) [@algunion](https://discourse.julialang.org/u/algunion)\
**Post date:** [August 4, 2023, 2:22am UTC](https://discourse.julialang.org/t/store-dataframes-in-structure-and-storing-multiple-database-connection-in-connection-pool/102467/2 "2023-08-04T02:22:51Z")

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You need to call a constructor first to get an instance of `all_dataframes1` (also, take a look at the naming conventions, it is pretty unconventional to use `all_dataframes1` as the type name).

In other words, if you want to mutate your `struct` object, you need to have a struct object in the first place:

```julia
# assuming you already have two DataFrame objects called df1 and df2
myobj = all_dataframes1(df1, df2)

# now you can mutate your fields if you need:
myobj.map_df = df3

```

Also, keep in mind that `Setfield.jl` (and `Accessors.jl`) are designated for updating immutable structs (and more), and you end up constructing new objects altogether - there is no reason for using these packages if you already have a `mutable` struct.

* * *

A few things about structs.

When defining a struct in the following way:

```julia
mutable struct A
    fieldX::Int
    fieldY::String
end

```

you cannot just go and do `A.fieldX = 1` right away. Instead, you need to call the default constructor:

```julia
a = A(1, "hello")

```

Now you are free to update the fields of the instance `a` of type `A`: `a.fieldX = 10` and `a.fieldY = "bye"`

You can read more about this [here](https://docs.julialang.org/en/v1/manual/types/#Composite-Types).

* * *

Now, to answer your second question (which is hidden in the code block, by mistake, I assume).

There is an old/archived package `ConnectionPools.jl`; you shouldn’t expect to find it in the registry (pre-Julia 1.0).

I am not aware of any generic connection pool-related package at this point. I suggest checking the documentation of the db-related packages you already use.

If somebody else has more information about the connection pools issue, I hope they will jump into the conversation and share more.

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<div class="post-metadata">

**Author:** ![rocco\_sprmnt21](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rocco_sprmnt21/32/20127_2.png) [@rocco\_sprmnt21](https://discourse.julialang.org/u/rocco_sprmnt21)\
**Post date:** [August 4, 2023, 9:14am UTC](https://discourse.julialang.org/t/store-dataframes-in-structure-and-storing-multiple-database-connection-in-connection-pool/102467/3 "2023-08-04T09:14:42Z")

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Have you considered storing dataframes as nested elements of a parent dataframe?  
I don’t know what kind of processing/access you have to do next, but the flexibility of the dataframes structure could be useful for your purposes, without “inventing” anything new.

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<div class="post-metadata">

**Author:** ![Sandy45](https://avatars.discourse-cdn.com/v4/letter/s/3ec8ea/32.png) [@Sandy45](https://discourse.julialang.org/u/Sandy45)\
**Post date:** [August 4, 2023, 2:14pm UTC](https://discourse.julialang.org/t/store-dataframes-in-structure-and-storing-multiple-database-connection-in-connection-pool/102467/4 "2023-08-04T14:14:19Z")

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@algunion Thanks for your clarification, Yes, that’s worked for me and I will try to read more about it.