Support as_int on bool series

docs/includes/generated_docs/language__series.md

@@ -119,6 +119,14 @@ status = status_code.map_values(
 ```
 </div>
 
+<div class="attr-heading" id="BoolPatientSeries.as_int">
+  <tt><strong>as_int</strong>()</tt>
+  <a class="headerlink" href="#BoolPatientSeries.as_int" title="Permanent link">🔗</a>
+</div>
+<div markdown="block" class="indent">
+Return each value in this Boolean series as 1 (True) or 0 (False).
+</div>
+
 </div>
 
 
@@ -243,6 +251,14 @@ status = status_code.map_values(
 ```
 </div>
 
+<div class="attr-heading" id="BoolEventSeries.as_int">
+  <tt><strong>as_int</strong>()</tt>
+  <a class="headerlink" href="#BoolEventSeries.as_int" title="Permanent link">🔗</a>
+</div>
+<div markdown="block" class="indent">
+Return each value in this Boolean series as 1 (True) or 0 (False).
+</div>
+
 <div class="attr-heading" id="BoolEventSeries.count_distinct_for_patient">
   <tt><strong>count_distinct_for_patient</strong>()</tt>
   <a class="headerlink" href="#BoolEventSeries.count_distinct_for_patient" title="Permanent link">🔗</a>

docs/includes/generated_docs/specs.md

@@ -2558,6 +2558,33 @@ returns the following patient series:
 
 
 
+### 7.2 Convert a boolean value to an integer
+
+
+#### 7.2.1 Bool as int
+Booleans are converted to 0 (False) or 1 (True).
+
+This example makes use of a patient-level table named `p` containing the following data:
+
+| patient|b1 |
+| - | - |
+| 1|T |
+| 2| |
+| 3|F |
+
+```python
+p.b1.as_int()
+```
+returns the following patient series:
+
+| patient | value |
+| - | - |
+| 1|1 |
+| 2| |
+| 3|0 |
+
+
+
 ## 8 Operations on integer series
 
 
@@ -3030,6 +3057,8 @@ returns the following patient series:
 
 
 #### 11.1.3 Case with boolean column
+Note that individual boolean columns can be converted to the integers 0 and 1 using
+the `as_int()` method.
 
 This example makes use of a patient-level table named `p` containing the following data:
 

ehrql/measures/calculate.py

@@ -4,7 +4,7 @@
 
 from ehrql.measures.measures import get_all_group_by_columns
 from ehrql.query_model.column_specs import ColumnSpec, get_column_spec_from_series
-from ehrql.query_model.nodes import Case, Dataset, Function, Value, get_series_type
+from ehrql.query_model.nodes import Dataset, Function, Value, get_series_type
 from ehrql.query_model.transforms import substitute_parameters
 
 
@@ -207,15 +207,7 @@ def series_as_int(series):
     if series_type is int:
         return series
     elif series_type is bool:
-        # TODO: This is definitely not the most efficient way to do this. We should
-        # extend the `CastToInt` operation to apply to boolean as well.
-        return Case(
-            {
-                Function.EQ(series, Value(True)): Value(1),
-                Function.EQ(series, Value(False)): Value(0),
-            },
-            default=None,
-        )
+        return Function.CastToInt(series)
     else:
         assert False
 

ehrql/query_engines/trino.py

@@ -93,13 +93,14 @@ def date_difference_in_days(self, end, start):
         )
 
     def cast_to_int(self, value):
-        # Trino's casting to int rounds away from zero. We need to round towards zero for
-        # consistency with other query engines.
-        rounded_towards_zero = sqlalchemy.case(
-            (value > 0, SQLFunction("FLOOR", value)),
-            else_=SQLFunction("CEILING", value),
-        )
-        return sqlalchemy.cast(rounded_towards_zero, sqlalchemy.Integer)
+        if isinstance(value.type, sqlalchemy.Numeric):
+            # Trino's casting to int rounds away from zero. We need to round towards zero for
+            # consistency with other query engines.
+            value = sqlalchemy.case(
+                (value > 0, SQLFunction("FLOOR", value)),
+                else_=SQLFunction("CEILING", value),
+            )
+        return sqlalchemy.cast(value, sqlalchemy.Integer)
 
     def truedivide(self, lhs, rhs):
         rhs_null_if_zero = SQLFunction("NULLIF", rhs, 0.0, type_=sqlalchemy.Float)

ehrql/query_language.py

@@ -449,6 +449,16 @@ def __invert__(self: T) -> T:
         """
         return _apply(qm.Function.Not, self)
 
+    @overload
+    def as_int(self: "PatientSeries") -> "IntPatientSeries": ...
+    @overload
+    def as_int(self: "EventSeries") -> "IntEventSeries": ...
+    def as_int(self):
+        """
+        Return each value in this Boolean series as 1 (True) or 0 (False).
+        """
+        return _apply(qm.Function.CastToInt, self)
+
 
 class BoolPatientSeries(BoolFunctions, PatientSeries):
     _type = bool

ehrql/query_model/nodes.py

@@ -376,7 +376,7 @@ class FloorDivide(Series[int]):
 
     # Casting numeric types
     class CastToInt(Series[int]):
-        source: Series[Numeric]
+        source: Series[Numeric] | Series[bool]
 
     class CastToFloat(Series[float]):
         source: Series[Numeric]

tests/spec/bool_series_ops/test_conversion.py

@@ -0,0 +1,25 @@
+from ..tables import p
+
+
+title = "Convert a boolean value to an integer"
+
+table_data = {
+    p: """
+         | b1
+       --+----
+       1 |  T
+       2 |
+       3 |  F
+       """,
+}
+
+
+def test_bool_as_int(spec_test):
+    """
+    Booleans are converted to 0 (False) or 1 (True).
+    """
+    spec_test(
+        table_data,
+        p.b1.as_int(),
+        {1: 1, 2: None, 3: 0},
+    )

tests/spec/case_expressions/test_case.py

@@ -54,6 +54,10 @@ def test_case_with_default(spec_test):
 
 
 def test_case_with_boolean_column(spec_test):
+    """
+    Note that individual boolean columns can be converted to the integers 0 and 1 using
+    the `as_int()` method.
+    """
     table_data = {
         p: """
               | i1 | b1

tests/spec/toc.py

@@ -40,6 +40,7 @@
     ],
     "bool_series_ops": [
         "test_logical_ops",
+        "test_conversion",
     ],
     "int_series_ops": [
         "test_arithmetic_ops",