Is it possible to abstract join, (and possibly filter, and select logic) into dedicated functions?

[dessalines]

I have a case where I need to build database views, (for example a PostView), made up of the same join, and do one of several things with the results of that join.

• Select a single row from that join, by an id.
• Select many rows from the same join, with filters.
• Select a count from that same join, with different filters.

Currently I have to copy-paste the code for the join for all three of these, but I'd really like to be able to abstract this logic into dedicated functions. For example:

fn view_joins() -> ;
fn filters(join_result, filters...) -> ;
fn selects(filtered_join_result), -> ;

The problem I'm having, is that the return type of the join is incredibly complicated (far too complicated to try to work out by hand), and rust-analyzer and clippy only give an output type using diesels internal types, not ones usable in my code base.

Is there any way around this, so that I can re-use the same join? Is there a generic / boxed type so that I can reuse this join elsewhere?

Thanks in advance.

[weiznich]

The Composing Applications Guide contains examples and explanations how to handle this kind of situations.

[dessalines]

Thx, I had no idea about a lot of these!

EDIT: For the record, to do this, add the following to a function that contains either join or filter logic:

#[diesel::dsl::auto_type]
fn joins() -> _ {
  ...
}

[dessalines]

EDIT: apologies, I just read your comment about AllColumns and will try that out now.

#[dsl::auto_type] is working really well for abstracting joins and filters into common functions.

Is there a way to similarly auto-type select statements? For example, if I have a select statement that selects all the columns from several of the joins, like so:

const SELECTION: SelectionType = (
  local_user::all_columns,
  local_user_vote_display_mode::all_columns,
  person::all_columns,
  person_aggregates::all_columns,
);

The SelectionType ends up having a very complicated type.

[dessalines]

For anyone looking for this in the future, you can do the following to simplify a select type from a join:

type SelectionType = (
<local_user::table as diesel::Table>::AllColumns,
... all the other joined columns

const SELECTION = SelectionType = (
  local_user::all_columns,
  ...