Add property matcher

README.org

@@ -66,6 +66,13 @@ Another way to look at it is like a "query language" for Org buffers.  For examp
             (and (not (done))
                  (tags "Emacs")
                  (priority >= "B"))))  ;; => (44154 46469 56008 63965 100008 ...)
+
+  ;; If you kept a database of music in an Org file, you might run a query like this to find tracks
+  ;; composed by Chopin that do not have their key recorded in the database:
+  (org-ql "~/org/music.org"
+    (and (property "genre" "classical")
+         (property "composer" "Chopin")
+         (not (property "key"))))
 #+END_SRC
 
 Instead of opening an agenda-like buffer with matching entries, =org-ql= can take a function as an argument that is called at each matching entry to return a result, and finally it returns a list of the results.  For example, you could return a list of positions within the buffer, or a list of headings, or headings with entry contents, etc.  By default, the matching element is returned, which is the result of calling =org-element-headline-parser= at that entry.

notes.org

@@ -186,6 +186,16 @@ Doesn't seem to make any difference.
     (regexp "over"))
 #+END_SRC
 
+** Property matching
+
+#+BEGIN_SRC elisp
+  (org-agenda-ng "~/src/emacs/org-super-agenda/test/test.org"
+    (property "agenda-group"))
+
+  (org-agenda-ng "~/src/emacs/org-super-agenda/test/test.org"
+    (property "agenda-group" "plans"))
+#+END_SRC
+
 ** Screenshot code
 
 #+BEGIN_SRC elisp

org-agenda-ng.el

@@ -128,6 +128,7 @@ NONE-PREDS."
                           (todo #'org-agenda-ng--todo-p)
                           (done #'org-agenda-ng--done-p)
                           (tags #'org-agenda-ng--tags-p)
+                          (property #'org-ql--property-p)
                           (regexp #'org-ql--regexp-p)
                           (org-back-to-heading #'outline-back-to-heading))
       (org-with-wide-buffer
@@ -474,3 +475,15 @@ comparator, PRIORITY should be a priority string."
     (save-excursion
       (goto-char (line-beginning-position))
       (re-search-forward regexp end t))))
+
+(defun org-ql--property-p (property &optional value)
+  "Return non-nil if current entry has PROPERTY, and optionally VALUE."
+  (pcase property
+    ('nil (user-error "Property matcher requires a PROPERTY argument."))
+    (_ (pcase value
+         ('nil
+          ;; Check that PROPERTY exists
+          (org-entry-get (point) property))
+         (_
+          ;; Check that PROPERTY has VALUE
+          (string-equal value (org-entry-get (point) property 'selective)))))))