Prototype support for named (pseudo-field) queries

As discussed here:
https://github.com/beetbox/beets/pull/3145#pullrequestreview-204523870

This would replace the need for #3149.

beets/dbcore/db.py

@@ -143,6 +143,11 @@ class Model(object):
     are subclasses of `Sort`.
     """
 
+    _queries = {}
+    """Named queries that use a field-like `name:value` syntax but which
+    do not relate to any specific field.
+    """
+
     _always_dirty = False
     """By default, fields only become "dirty" when their value actually
     changes. Enabling this flag marks fields as dirty even when the new

beets/dbcore/queryparse.py

@@ -119,12 +119,13 @@ def construct_query_part(model_cls, prefixes, query_part):
     if not query_part:
         return query.TrueQuery()
 
-    # Use `model_cls` to build up a map from field names to `Query`
-    # classes.
+    # Use `model_cls` to build up a map from field (or query) names to
+    # `Query` classes.
     query_classes = {}
     for k, t in itertools.chain(model_cls._fields.items(),
                                 model_cls._types.items()):
         query_classes[k] = t.query
+    query_classes.update(model_cls._queries)  # Non-field queries.
 
     # Parse the string.
     key, pattern, query_class, negate = \
@@ -137,26 +138,27 @@ def construct_query_part(model_cls, prefixes, query_part):
             # The query type matches a specific field, but none was
             # specified. So we use a version of the query that matches
             # any field.
-            q = query.AnyFieldQuery(pattern, model_cls._search_fields,
-                                    query_class)
-            if negate:
-                return query.NotQuery(q)
-            else:
-                return q
+            out_query = query.AnyFieldQuery(pattern, model_cls._search_fields,
+                                            query_class)
         else:
             # Non-field query type.
-            if negate:
-                return query.NotQuery(query_class(pattern))
-            else:
-                return query_class(pattern)
+            out_query = query_class(pattern)
 
-    # Otherwise, this must be a `FieldQuery`. Use the field name to
-    # construct the query object.
-    key = key.lower()
-    q = query_class(key.lower(), pattern, key in model_cls._fields)
+    # Field queries get constructed according to the name of the field
+    # they are querying.
+    elif issubclass(query_class, query.FieldQuery):
+        key = key.lower()
+        out_query = query_class(key.lower(), pattern, key in model_cls._fields)
+
+    # Non-field (named) query.
+    else:
+        out_query = query_class(pattern)
+
+    # Apply negation.
     if negate:
-        return query.NotQuery(q)
-    return q
+        return query.NotQuery(out_query)
+    else:
+        return out_query
 
 
 def query_from_strings(query_cls, model_cls, prefixes, query_parts):

test/test_dbcore.py

@@ -36,6 +36,17 @@ class TestSort(dbcore.query.FieldSort):
     pass
 
 
+class TestQuery(dbcore.query.Query):
+    def __init__(self, pattern):
+        self.pattern = pattern
+
+    def clause(self):
+        return None, ()
+
+    def match(self):
+        return True
+
+
 class TestModel1(dbcore.Model):
     _table = 'test'
     _flex_table = 'testflex'
@@ -49,6 +60,9 @@ class TestModel1(dbcore.Model):
     _sorts = {
         'some_sort': TestSort,
     }
+    _queries = {
+        'some_query': TestQuery,
+    }
 
     @classmethod
     def _getters(cls):
@@ -519,6 +533,10 @@ class QueryFromStringsTest(unittest.TestCase):
         q = self.qfs([''])
         self.assertIsInstance(q.subqueries[0], dbcore.query.TrueQuery)
 
+    def test_parse_named_query(self):
+        q = self.qfs(['some_query:foo'])
+        self.assertIsInstance(q.subqueries[0], TestQuery)
+
 
 class SortFromStringsTest(unittest.TestCase):
     def sfs(self, strings):