Use existing m2m table if one exists

sqlite_utils/db.py

@@ -1069,7 +1069,22 @@ class Table:
             assert record_or_list is not None, "Provide lookup= or record, not both"
         tables = list(sorted([self.name, other_table]))
         columns = ["{}_id".format(t) for t in tables]
-        m2m_table_name = m2m_table or "{}_{}".format(*tables)
+        if m2m_table is not None:
+            m2m_table_name = m2m_table
+        else:
+            # Detect if there is a single, unambiguous option
+            candidates = self.db.m2m_table_candidates(self.name, other_table)
+            if len(candidates) == 1:
+                m2m_table_name = candidates[0]
+            elif len(candidates) > 1:
+                raise NoObviousTable(
+                    "No single obvious m2m table for {}, {} - use m2m_table= parameter".format(
+                        self.name, other_table
+                    )
+                )
+            else:
+                # If not, create a new table
+                m2m_table_name = m2m_table or "{}_{}".format(*tables)
         m2m_table = self.db.table(m2m_table_name, pk=columns, foreign_keys=columns)
         if lookup is None:
             records = (

tests/test_m2m.py

@@ -1,4 +1,4 @@
-from sqlite_utils.db import ForeignKey
+from sqlite_utils.db import ForeignKey, NoObviousTable
 import pytest
 
 
@@ -107,12 +107,37 @@ def test_m2m_table_candidates(fresh_db):
 
 
 def test_uses_existing_m2m_table_if_exists(fresh_db):
-    # Code should look for an existing toble with fks to both tables
+    # Code should look for an existing table with fks to both tables
     # and use that if it exists.
-    assert False
+    people = fresh_db.create_table("people", {"id": int, "name": str}, pk="id")
+    fresh_db["tags"].lookup({"tag": "Coworker"})
+    fresh_db.create_table(
+        "tagged",
+        {"people_id": int, "tags_id": int},
+        foreign_keys=["people_id", "tags_id"],
+    )
+    people.insert({"name": "Wahyu"}).m2m("tags", lookup={"tag": "Coworker"})
+    assert fresh_db["tags"].exists
+    assert fresh_db["tagged"].exists
+    assert not fresh_db["people_tags"].exists
+    assert not fresh_db["tags_people"].exists
+    assert [{"people_id": 1, "tags_id": 1}] == list(fresh_db["tagged"].rows)
 
 
 def test_requires_explicit_m2m_table_if_multiple_options(fresh_db):
     # If the code scans for m2m tables and finds more than one candidate
     # it should require that the m2m_table=x argument is used
-    assert False
+    people = fresh_db.create_table("people", {"id": int, "name": str}, pk="id")
+    fresh_db["tags"].lookup({"tag": "Coworker"})
+    fresh_db.create_table(
+        "tagged",
+        {"people_id": int, "tags_id": int},
+        foreign_keys=["people_id", "tags_id"],
+    )
+    fresh_db.create_table(
+        "tagged2",
+        {"people_id": int, "tags_id": int},
+        foreign_keys=["people_id", "tags_id"],
+    )
+    with pytest.raises(NoObviousTable):
+        people.insert({"name": "Wahyu"}).m2m("tags", lookup={"tag": "Coworker"})