I am pretty new to Rust and just trying to learn it while coding a GraphQL API using MongoDB. Currently, I am struggling with decoding a document into my CourseDocument struct.
#[derive(Serialize, Deserialize, Debug, Eq)]
struct CourseDocument {
#[serde(rename = "_id")]
id: ObjectId,
#[serde(rename = "localizedFields")]
localized_fields: Vec<CourseDocumentLocalizedFields>,
categories: Vec<ObjectId>,
tags: Vec<ObjectId>,
trainers: Vec<ObjectId>,
videos: Vec<ObjectId>,
}
impl PartialEq for CourseDocument {
fn eq(&self, other: &Self) -> bool {
self.id == other.id
}
}
In my main() function I tried the following
let course_collection = database.collection::<CourseDocument>("courses");
let projection = doc! {"localizedFields": 1};
let options = FindOptions::builder()
.limit(10)
.projection(projection)
.build();
let mut cursor = course_collection.find(None, options).await.unwrap();
while let Some(course) = cursor.try_next().await.unwrap() {
println!("{:#?}", course)
}
This code throws the following error:
Error { kind: InvalidResponse { message: "missing field 'categories'" }, labels: {} }
The error does make sense because the CourseDocument requires the categories field to be at least an empty vector. But I am still wondering what would be the correct struct declaration.
Do I have to wrap every field with an Option enum to make projections and typed documents possible?
You can use clone_with_type() to make a Collection with the same source but with a different type. Using that, you can deserialize into a type with only those projected fields.
#[derive(Serialize, Deserialize)]
struct ProjectedCourseDocument {
#[serde(rename = "localizedFields")]
localized_fields: Vec<CourseDocumentLocalizedFields>,
}
let mut cursor = course_collection
.clone_with_type::<ProjectedCourseDocument>()
.find(None, options)
.await
.unwrap();