sea-orm-codegen: Enum use statements getting generated multiple times

[lukasschlueter]

Description

When using PostgreSQL, a table that has two columns using the same enum causes the generated entity to have duplicate use statements, which cause a compilation error.

Steps to Reproduce

1. Create a migration like this:

use sea_orm_migration::prelude::*;
use sea_orm_migration::prelude::extension::postgres::Type;
use sea_orm_migration::sea_orm::DbBackend;

#[derive(DeriveMigrationName)]
pub struct Migration;

#[async_trait::async_trait]
impl MigrationTrait for Migration {
    async fn up(&self, manager: &SchemaManager) -> Result<(), DbErr> {
        assert_eq!(manager.get_database_backend(), DbBackend::Postgres);
        manager.create_type(Type::create().as_enum(MyEnum::Table).values([MyEnum::A, MyEnum::B]).to_owned()).await?;

        manager
            .create_table(
                Table::create()
                    .table(MyTable::Table)
                    .if_not_exists()
                    .col(
                        ColumnDef::new(MyTable::Id)
                            .integer()
                            .not_null()
                            .auto_increment()
                            .primary_key(),
                    )
                    .col(ColumnDef::new(MyTable::Col1).enumeration(MyEnum::Table, [MyEnum::A, MyEnum::B]).not_null())
                    .col(ColumnDef::new(MyTable::Col2).enumeration(MyEnum::Table, [MyEnum::A, MyEnum::B]).not_null())
                    .to_owned(),
            )
            .await
    }

    async fn down(&self, manager: &SchemaManager) -> Result<(), DbErr> {
        manager
            .drop_table(Table::drop().table(MyTable::Table).to_owned())
            .await
    }
}

/// Learn more at https://docs.rs/sea-query#iden
#[derive(Iden)]
enum MyTable {
    Table,
    Id,
    Col1,
    Col2,
}

#[derive(Iden)]
enum MyEnum {
    Table,
    A,
    B,
}

2. Migrate: sea migrate
3. Generate entities: sea generate entity -o src/entities
4. Have a look at the generated file (in this case: src/entities/my_table.rs)

Expected Behavior

The generated file has only one import and compiles without errors.
Generated file should look like this:

//! `SeaORM` Entity. Generated by sea-orm-codegen 0.12.0

use super::sea_orm_active_enums::MyEnum;
use sea_orm::entity::prelude::*;

/* ... */

Actual Behavior

Generated file has duplicate use statements and looks like this:

//! `SeaORM` Entity. Generated by sea-orm-codegen 0.12.0

use super::sea_orm_active_enums::MyEnum;
use super::sea_orm_active_enums::MyEnum;
use sea_orm::entity::prelude::*;

/* ... */

Reproduces How Often

Always

Workarounds

1. Manually remove the duplicate use statement each time

2. I've implemented a simple fix in 7cf3195

Note: I tried to fix this and manage to solve it for my project where the bug originally showed up.
However, I would have liked to add some kind of tests for that but I didn't really understand how. - https://github.com/SeaQL/sea-orm/blob/master/CONTRIBUTING.md did state how to run all the tests (which were successful for my commit) but not how the test system works and where to add my own tests.
I currently don't have the time to try to understand how tests work, so unfortunately, I don't think it would be much help if I opened a PR myself (or would it?).

So if someone wants to pick this up - feel free to use my changes, the main thing that's missing is some test.

Reproducible Example

Currently, it does not seem like I will have time to add a PR for that unfortunately.

Versions

sea-orm-cli: master (1431d80)
Postgres (14.5 - should not matter)
Windows 10 (should not matter)

[billy1624]

Hey @lukasschlueter, nice catch and neat little patch! A PR would be welcomed! Could you please file a PR when you got time?