Add PaginatorTrait and CountTrait for more constrains

[YoshieraHuang]

When I build my web api using sea-orm, I want to constrain my select type to have paginate() method. So I added a PaginatorTrait and move the paginate() method to this trait. Meanwhile count() method of select type depends on paginate() method, but not all paginateable select has this method. So I added another CountTrait trait.

My use case of PaginatorTrait is like this:

// Data transfer object, aka dto
#[derive(Debug, Serialize)]
pub struct Paginated<T> {
    items: Vec<T>,
    page_size: usize,
    page_index: usize,
    num_pages: usize,
    num_items: usize,
    has_previous: bool,
    has_next: bool,
}

impl<T> Paginated<T>
{
    pub async fn query_with_map<'db, P, S, F>(
        db: &'db DatabaseConnection,
        paginate: P,
        page_size: usize,
        page_index: usize,
        predicate: F)
    -> Result<Self, DbErr>
    where
        P: PaginatorTrait<'db, DatabaseConnection, Selector = S>,
        S: SelectorTrait + 'db,
        F: FnMut(<S as SelectorTrait>::Item) -> T
    {
        let paginator = paginate.paginate(db, page_size);
        let original_items = paginator.fetch_page(page_index - 1).await?;
        let items = original_items.into_iter().map(predicate).collect();
        let num_pages = paginator.num_pages().await?;
        let num_items = paginator.num_items().await?;
        let has_previous = num_pages > 0 && page_index > 1;
        let has_next = num_pages > 0 && page_index < (num_pages - 1);
        Ok(Paginated {
            items,
            page_size,
            page_index,
            num_pages,
            num_items,
            has_previous,
            has_next
        })
    }
}

These traits may be used not so often but needed for me. It will be fine if this PR is merged.

By the way, if this PR will be merged, feel free to modify the docs and variable names, cause i'm not a native english speaker.

[tyt2y3]

Thank you so much for your contribution, again!
Can you point out which Struct has paginate but not count?
I think we should fix them indeed.

[YoshieraHuang]

Thank you so much for your contribution, again! Can you point out which Struct has paginate but not count? I think we should fix them indeed.

Selector struct has paginate but not count. Select and SelectTwo have both method but in their paginate method, they are converted to Selector and then do pagination. SelectTwoMany does not have both since its pagination is not easy. I think implementing count for Selector is not bad. I will move count into PaginatorTrait.

[billy1624]

OMG... I'm sorry didn't mean to merge this now

[tyt2y3]

Open a new PR or if it's minor just push onto master