update documentation to note that both -> and _ are supported by default as a delimiter

Author: tmaiaroto

DESIGN_PHILOSOPHIES.md

@@ -45,3 +45,12 @@ Making a guess might seem helpful, but it can hide serious, silent bugs. The fol
 -   **Destination:** `var blogs []BlogWithPosts`
 -   **Behavior:** `carta` **gracefully handles** the fact that the same blog ID appears in multiple rows. It creates one `Blog` object and appends each unique `Post` to its `Posts` slice.
 -   **Why this is Graceful:** This is the core purpose of the library. There is no ambiguity. The library uses the unique ID of the `Blog` (the `b.id` column) to understand that these rows all describe the same parent entity. This is a well-defined transformation, not a guess.
+
+---
+
+### Scenario 4: Default `_` Delimiter Support (Graceful Handling)
+
+-   **Query:** `SELECT b.id, a.id AS "author_id" FROM blogs b JOIN authors a ON b.author_id = a.id`
+-   **Destination:** `var blogs []BlogWithAuthor`
+-   **Behavior:** `carta` **gracefully handles** the `author_id` column, correctly mapping it to the `Author` struct's `id` field, even though the default delimiter is `->`.
+-   **Why this is Graceful:** This convenience was a deliberate design choice. Since SQL `SELECT` statements must have unambiguous column names (which you control with aliases), there is no risk of conflict with actual database field names that contain underscores. This allows for more natural-looking column names in queries without requiring an explicit `delimiter=_` option in the `carta` tag. If another delimiter is desired, it must be set explicitly.

README.md

@@ -134,7 +134,7 @@ type User struct {
 ```
 
 #### Associations (Nested Structs)
-For nested structs (has-one or has-many relationships), use the `carta` tag to define a prefix for the nested struct's columns. The default delimiter between the prefix and the field name is `->`.
+For nested structs (has-one or has-many relationships), use the `carta` tag to define a prefix for the nested struct's columns. The default delimiter is `->`, but `_` is also supported out-of-the-box for convenience. This dual support does not create conflicts with field names containing underscores (e.g., `first_name`) because mapping is based on the final, unambiguous column names returned by your `SELECT` query, which you control via SQL aliases.
 
 **Example:**
 ```go
@@ -165,17 +165,17 @@ left join authors a on b.author_id = a.id
 This design promotes struct reusability. The `Author` struct can be used on its own to map to a query like `select id, username from authors` or as a nested struct within `Blog` as shown above.
 
 **Custom Delimiter:**
-You can override the default delimiter by specifying it in the `carta` tag.
+If you need to use a delimiter other than the default `->` or the supported `_`, you can override it by specifying it in the `carta` tag.
 ```go
 type Blog struct {
-    Author Author `carta:"author,delimiter=_"`
+    Author Author `carta:"author,delimiter=."` // Using a dot as a delimiter
 }
 ```
 **Corresponding SQL Query:**
 ```sql
 select
-    a.id as "author_id",
-    a.username as "author_username"
+    a.id as "author.id",
+    a.username as "author.username"
 ...
 ```