refactor composer to use sqlalchemy, avoid string concatenations and follow conventions

api/composer/metrics.py

@@ -0,0 +1,30 @@
+"""Metric-fragment qualification.
+
+Metric expressions in `metrics.yaml` are written *unaliased* — e.g.
+`AVG(CASE WHEN status = 'B' THEN 1.0 WHEN status = 'A' THEN 0.0 END)`, `A13`,
+`SUM(amount)`. Before such a fragment can sit in a multi-table query it has to
+be bound to the metric's own table alias, so the composer can introduce other
+tables via joins without ambiguity.
+
+We parse the fragment with sqlglot and inject the alias as the table qualifier
+on every bare column reference, returning the resulting expression node (not a
+string) so it splices directly into the query tree the composer assembles.
+"""
+
+from __future__ import annotations
+
+import sqlglot
+from sqlglot import exp
+
+
+def qualify_metric_expr(sql_fragment: str, table_alias: str) -> exp.Expression:
+    """Parse `sql_fragment` and qualify its bare column refs to `table_alias`.
+
+    Used for both the metric's SELECT expression and its optional WHERE filter
+    (both are written as bare expressions over the metric's table)."""
+    tree = sqlglot.parse_one(sql_fragment, dialect="postgres")
+    for col in tree.find_all(exp.Column):
+        if col.table:
+            continue
+        col.set("table", exp.Identifier(this=table_alias, quoted=True))
+    return tree