# Objective
Currently scenes define components using a list:
```rust
[
(
entity: 0,
components: [
{
"bevy_transform::components::transform::Transform": (
translation: (
x: 0.0,
y: 0.0,
z: 0.0
),
rotation: (0.0, 0.0, 0.0, 1.0),
scale: (
x: 1.0,
y: 1.0,
z: 1.0
),
),
},
{
"my_crate::Foo": (
text: "Hello World",
),
},
{
"my_crate::Bar": (
baz: 123,
),
},
],
),
]
```
However, this representation has some drawbacks (as pointed out by @Metadorius in [this](https://github.com/bevyengine/bevy/pull/4561#issuecomment-1202215565) comment):
1. Increased nesting and more characters (minor effect on overall size)
2. More importantly, by definition, entities cannot have more than one instance of any given component. Therefore, such data is best stored as a map— where all values are meant to have unique keys.
## Solution
Change `components` to store a map of components rather than a list:
```rust
[
(
entity: 0,
components: {
"bevy_transform::components::transform::Transform": (
translation: (
x: 0.0,
y: 0.0,
z: 0.0
),
rotation: (0.0, 0.0, 0.0, 1.0),
scale: (
x: 1.0,
y: 1.0,
z: 1.0
),
),
"my_crate::Foo": (
text: "Hello World",
),
"my_crate::Bar": (
baz: 123
),
},
),
]
```
#### Code Representation
This change only affects the scene format itself. `DynamicEntity` still stores its components as a list. The reason for this is that storing such data as a map is not really needed since:
1. The "key" of each value is easily found by just calling `Reflect::type_name` on it
2. We should be generating such structs using the `World` itself which upholds the one-component-per-entity rule
One could in theory create manually create a `DynamicEntity` with duplicate components, but this isn't something I think we should focus on in this PR. `DynamicEntity` can be broken in other ways (i.e. storing a non-component in the components list), and resolving its issues can be done in a separate PR.
---
## Changelog
* The scene format now uses a map to represent the collection of components rather than a list
## Migration Guide
The scene format now uses a map to represent the collection of components. Scene files will need to update from the old list format.
<details>
<summary>Example Code</summary>
```rust
// OLD
[
(
entity: 0,
components: [
{
"bevy_transform::components::transform::Transform": (
translation: (
x: 0.0,
y: 0.0,
z: 0.0
),
rotation: (0.0, 0.0, 0.0, 1.0),
scale: (
x: 1.0,
y: 1.0,
z: 1.0
),
),
},
{
"my_crate::Foo": (
text: "Hello World",
),
},
{
"my_crate::Bar": (
baz: 123,
),
},
],
),
]
// NEW
[
(
entity: 0,
components: {
"bevy_transform::components::transform::Transform": (
translation: (
x: 0.0,
y: 0.0,
z: 0.0
),
rotation: (0.0, 0.0, 0.0, 1.0),
scale: (
x: 1.0,
y: 1.0,
z: 1.0
),
),
"my_crate::Foo": (
text: "Hello World",
),
"my_crate::Bar": (
baz: 123
),
},
),
]
```
</details>
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