Changed name generation for join and column aliases

This will make sql statements more compact but harder to read.
More importantly, it fixes the issue of aliases becoming larger
than postgres allows.

changelog.txt

@@ -11,6 +11,7 @@ Since 0.6.3
 - improved error spans in or! and and!
 - removed `AsDbType::from_primitive`
 - fixed names of join aliases
+- changed name generation for join and column aliases
 
 Notes for publishing
 --------------------

src/internal/query_context/mod.rs

@@ -2,6 +2,8 @@
 
 use std::borrow::Cow;
 use std::collections::HashMap;
+use std::fmt;
+use std::fmt::Write;
 
 use rorm_db::sql::join_table::JoinType;
 use rorm_db::sql::ordering::Ordering;
@@ -38,11 +40,7 @@ impl<'v> QueryContext<'v> {
 
     /// Add a field to select returning its index and alias
     pub fn select_field<F: Field, P: Path>(&mut self) -> (usize, String) {
-        let alias = format!(
-            "{path}__{field}",
-            path = P::add_to_context(self),
-            field = F::NAME
-        );
+        let alias = format!("{}", NumberAsAZ(self.selects.len()));
         self.selects.push(Select {
             table_name: PathId::of::<P>(),
             column_name: F::NAME,
@@ -54,12 +52,7 @@ impl<'v> QueryContext<'v> {
 
     /// Add a field to aggregate returning its index and alias
     pub fn select_aggregation<A: AggregationFunc, F: Field, P: Path>(&mut self) -> (usize, String) {
-        let alias = format!(
-            "{path}__{field}___{func}",
-            path = P::add_to_context(self),
-            field = F::NAME,
-            func = A::NAME,
-        );
+        let alias = format!("{}", NumberAsAZ(self.selects.len()));
         self.selects.push(Select {
             table_name: PathId::of::<P>(),
             column_name: F::NAME,
@@ -238,11 +231,7 @@ impl<'v> QueryContext<'v> {
     {
         let path_id = PathId::of::<P::Step<F>>();
         if !self.join_aliases.contains_key(&path_id) {
-            let alias = format!(
-                "{field}__{path}",
-                field = F::NAME,
-                path = P::add_to_context(self)
-            );
+            let alias = format!("{}", NumberAsAZ(self.join_aliases.len()));
             self.join_aliases.insert(path_id, alias);
             self.joins.push({
                 Join {
@@ -282,3 +271,25 @@ struct OrderBy {
     table_name: PathId,
     ordering: Ordering,
 }
+
+/// Adapter to display a number using the alphabet as digits
+struct NumberAsAZ(usize);
+impl fmt::Display for NumberAsAZ {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        static ALPHABET: [char; 26] = [
+            'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q',
+            'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z',
+        ];
+        let mut x = self.0;
+        match x {
+            0..26 => f.write_char(ALPHABET[x]),
+            _ => {
+                while x > 26 {
+                    f.write_char(ALPHABET[x % 26])?;
+                    x /= 26;
+                }
+                f.write_char(ALPHABET[x])
+            }
+        }
+    }
+}