feat: add open-existing-db example to address issue #20

Adds a new example that demonstrates ObjectBox transparently opens an
existing database when Store(directory=...) points to a directory that
already contains a data.mdb file. No special API is required.

The example covers a common question from users: how to open a database
that was created by a previous run or by another ObjectBox SDK.

Also updates example/README.md to list the new example.

Closes #20

Author: ElizioMartins

example/README.md

@@ -13,6 +13,7 @@ The following examples are available from this directory:
 
 - `tasks`: CRUD Example (see below for details)
 - `vectorsearch-cities`: VectorSearch Example (see below for details)
+- `open-existing-db`: Opening an Existing Database Example (see below for details)
 - `ollama`: LLM + VectorSearch Embeddings Script Example (See [ollama/README.md](./ollama/README.md) for details)
 
 
@@ -94,3 +95,34 @@ ID   Name                        Latitude  Longitude  Score
 164  San Salvador                   13.69     -89.22  1261.12
  67  Havana                         23.11     -82.37  1317.73
 ```
+
+## Example: Opening an Existing Database
+
+This example shows that `Store(directory="...")` **opens an existing database transparently** — no special API is required. If the directory already contains an ObjectBox database (`data.mdb`), it is opened; otherwise a new one is created.
+
+This is useful when:
+- You want to read a database written by a previous run of your program
+- You receive a database file created by another ObjectBox SDK (e.g. Android/Java, Swift, Dart) and want to read it with Python using the same entity model
+
+```
+cd open-existing-db
+
+# Step 1: write sample data (creates the database)
+$ python main.py write
+Wrote 3 notes to 'notes-db'.
+Run 'python main.py read' to open the same database and list them.
+
+# Step 2: open the SAME database and read it back
+$ python main.py read
+Found 3 note(s) in 'notes-db':
+
+ ID  Priority  Title                           Body
+----------------------------------------------------------------------
+  1         1  Buy groceries                   Milk, eggs, bread
+  2         2  Read ObjectBox docs             https://docs.objectbox.io
+  3         3  Open-source contribution        Submit a pull request today!
+
+# Reset: delete the database so you can start fresh
+$ python main.py reset
+Removed 'notes-db'. Run 'python main.py write' to start fresh.
+```

example/open-existing-db/main.py

@@ -0,0 +1,106 @@
+"""Example: Opening an Existing ObjectBox Database
+
+This example demonstrates that ObjectBox transparently opens an existing
+database when the given directory already contains one (data.mdb), and
+creates a new one if it doesn't. This means any program that uses the
+same entity model can read a database written by another program or even
+another ObjectBox SDK (e.g. Android/Java, Swift, Dart), as long as the
+model is compatible.
+
+Usage:
+    # Step 1 – write some data (creates the database on disk)
+    python main.py write
+
+    # Step 2 – open the same database and read the data back
+    python main.py read
+
+    # Reset: remove the database directory and start fresh
+    python main.py reset
+"""
+
+import os
+import sys
+import shutil
+
+from objectbox import Entity, Id, Int32, String, Store
+
+DB_DIRECTORY = "notes-db"
+
+
+@Entity()
+class Note:
+    id = Id()
+    title = String()
+    body = String()
+    priority = Int32()
+
+
+def open_store() -> Store:
+    """Open (or create) the ObjectBox store in DB_DIRECTORY.
+
+    ObjectBox will:
+    - Create the directory and a brand-new database if it does not exist yet.
+    - Open the existing database transparently if it already exists.
+    """
+    return Store(directory=DB_DIRECTORY)
+
+
+def write_data():
+    """Insert sample notes into the database."""
+    store = open_store()
+    box = store.box(Note)
+
+    notes = [
+        Note(title="Buy groceries", body="Milk, eggs, bread", priority=1),
+        Note(title="Read ObjectBox docs", body="https://docs.objectbox.io", priority=2),
+        Note(title="Open-source contribution", body="Submit a pull request today!", priority=3),
+    ]
+
+    for note in notes:
+        box.put(note)
+
+    print(f"Wrote {len(notes)} notes to '{DB_DIRECTORY}'.")
+    print("Run 'python main.py read' to open the same database and list them.")
+    store.close()
+
+
+def read_data():
+    """Open the existing database and print all stored notes."""
+    if not os.path.isdir(DB_DIRECTORY):
+        print(f"Database directory '{DB_DIRECTORY}' not found.")
+        print("Run 'python main.py write' first to create and populate it.")
+        sys.exit(1)
+
+    store = open_store()
+    box = store.box(Note)
+    notes = box.get_all()
+
+    if not notes:
+        print("No notes found in the database.")
+    else:
+        print(f"Found {len(notes)} note(s) in '{DB_DIRECTORY}':\n")
+        print(f"{'ID':>3}  {'Priority':>8}  {'Title':<30}  Body")
+        print("-" * 70)
+        for note in notes:
+            print(f"{note.id:>3}  {note.priority:>8}  {note.title:<30}  {note.body}")
+
+    store.close()
+
+
+def reset_data():
+    """Delete the database directory so the example can be run from scratch."""
+    if os.path.isdir(DB_DIRECTORY):
+        shutil.rmtree(DB_DIRECTORY)
+        print(f"Removed '{DB_DIRECTORY}'. Run 'python main.py write' to start fresh.")
+    else:
+        print(f"Nothing to reset – '{DB_DIRECTORY}' does not exist.")
+
+
+if __name__ == "__main__":
+    commands = {"write": write_data, "read": read_data, "reset": reset_data}
+
+    if len(sys.argv) != 2 or sys.argv[1] not in commands:
+        print("Usage: python main.py <write|read|reset>")
+        sys.exit(1)
+
+    commands[sys.argv[1]]()