fix: make column.unnest().topk() work

Also simplifies the implementation of Column.topk() and
makes it re-use the implementation of Table.topk()

This does NOT fix `column.unnest().topk(by=<anything besides None>)`. This is a rare case so I decided to just punt on implementing it

Author: NickCrews

ibis/backends/tests/test_generic.py

@@ -16,6 +16,7 @@
 import ibis.expr.datatypes as dt
 import ibis.selectors as s
 from ibis import _
+from ibis.backends.tests.conftest import NO_ARRAY_SUPPORT
 from ibis.backends.tests.errors import (
     ClickHouseDatabaseError,
     ExaQueryError,
@@ -2390,6 +2391,38 @@ def test_topk_counts_null(con):
     assert result[0].as_py() == 1
 
 
+@NO_ARRAY_SUPPORT
+def test_topk_unnest_count(con: ibis.BaseBackend):
+    t = con.create_table(
+        ibis.util.gen_name("topk_unnest_count"),
+        {"x": [[1, 2, 3], [1, 2, None], []]},
+        temp=True,
+    )
+    tk = t.x.unnest().topk(name="n")
+    n_1s = tk.filter(_.x == 1)["n"].as_scalar()
+    result = con.to_pyarrow(n_1s).as_py()
+    assert result == 2
+
+    tk = t.x.unnest().topk()
+    n_1s = tk.filter(_.x == 1)["x_count"].as_scalar()
+    result = con.to_pyarrow(n_1s).as_py()
+    assert result == 2
+
+
+@pytest.mark.xfail(reason="The unnest is not placed in the right place in the query")
+def test_topk_unnest_max(con: ibis.BaseBackend):
+    t = con.create_table(
+        ibis.util.gen_name("topk_counts_unnest"),
+        {"x": [[1, 2, 3], [1, 2, None], []]},
+        temp=True,
+    )
+    v = t.x.unnest()
+    tk = v.topk(by=v.max(), name="n")
+    n_1s = tk.filter(_.x == 1)["n"].as_scalar()
+    result = con.to_pyarrow(n_1s).as_py()
+    assert result == 1
+
+
 @pytest.mark.notyet(
     "clickhouse",
     raises=AssertionError,

ibis/expr/types/generic.py

@@ -2238,18 +2238,14 @@ def topk(
         """
         from ibis.expr.types.relations import bind
 
+        if by is None:
+            return self.as_table().topk(k=k, name=name)
+
         try:
-            (table,) = self.op().relations
+            (table_op,) = self.op().relations
         except ValueError:
             raise com.IbisTypeError("TopK must depend on exactly one table.")
-
-        table = table.to_expr()
-
-        if by is None and name is None:
-            # if `by` is something more complex, the _count doesn't make sense.
-            name = f"{self.get_name()}_count"
-        if by is None:
-            by = lambda t: t.count()
+        table: ibis.Table = table_op.to_expr()
 
         (metric,) = bind(table, by)
         if name is not None: