Add QuerySelect::columns method - select multiple columns (SeaQL/se…

…a-orm#1264)

SeaORM/docs/08-advanced-query/01-custom-select.md

@@ -18,7 +18,7 @@ assert_eq!(
 
 ## Select Some Attributes Only
 
-Use `select_only` and `column` methods together to select only the attributes you want.
+Use `select_only` and `column` methods together to select only the attribute you want.
 
 ```rust
 // Selecting the name column only
@@ -32,6 +32,35 @@ assert_eq!(
 );
 ```
 
+If you want to select multiple attributes at once.
+
+```rust
+assert_eq!(
+    cake::Entity::find()
+        .select_only()
+        .columns([cake::Column::Id, cake::Column::Name])
+        .build(DbBackend::Postgres)
+        .to_string(),
+    r#"SELECT "cake"."id", "cake"."name" FROM "cake""#
+);
+```
+
+Conditionally select all columns expect a specific column.
+
+```rust
+assert_eq!(
+    cake::Entity::find()
+        .select_only()
+        .columns(cake::Column::iter().filter(|col| match col {
+            cake::Column::Id => false,
+            _ => true,
+        }))
+        .build(DbBackend::Postgres)
+        .to_string(),
+    r#"SELECT "cake"."name" FROM "cake""#
+);
+```
+
 ## Select Custom Expressions
 
 Select any custom expression with `column_as` method, it takes any [`sea_query::SimpleExpr`](https://docs.rs/sea-query/*/sea_query/expr/enum.SimpleExpr.html) and an alias. Use [`sea_query::Expr`](https://docs.rs/sea-query/*/sea_query/expr/struct.Expr.html) helper to build `SimpleExpr`.