[FIX] core: optimize first snaphot in onchange

This patch speeds up the performance of method `onchange`.  Our use-case
is an invoice with 300 lines, where we modify the unit price or quantity
on several lines.  Each modification triggers a call to `onchange` on
the invoice itself.  The latter call went from 1.3 to 0.8 seconds, which
represents a speedup of 30% to 40%.

In the implementation of `onchange`, the first snapshot is preceded by a
"prefetching" phase, where the lines of x2many fields are read, so that
the fields of unmodified lines are in cache.  This is useful because the
fields of those lines are not sent by the client, which only sends ids.
This prefetching represents more than 40% of the duration of `onchange`,
in our use-case.

The prefetching is inefficient for several reasons.  First, it uses
`mapped`, which formats data that is actually never used.  Second, it
accesses new records that have the actual lines as origin.  And on those
records, computed stored fields are not taken from the origin record,
but are (uselessly) computed instead.

We have optimized the prefetching in the following way.  It now reads
stored fields on the actual lines (using `_read` to avoid formatting),
then copies the cache of those fields on the corresponding new lines (to
avoid useless computations).  This makes the prefetching about 10 times
faster!

closes odoo/odoo#59781

X-original-commit: ec50c426d7c2a17c7b72e64adab65eda6328e163
Signed-off-by: Raphael Collet (rco) <rco@openerp.com>

odoo/models.py

@@ -6065,20 +6065,34 @@ Fields:
                 if name not in values
             ]))
 
-        # prefetch x2many lines without data (for the initial snapshot)
+        # prefetch x2many lines: this speeds up the initial snapshot by avoiding
+        # to compute fields on new records as much as possible, as that can be
+        # costly and is not necessary at all
         for name, subnames in nametree.items():
             if subnames and values.get(name):
-                # retrieve all ids in commands
+                # retrieve all line ids in commands
                 line_ids = set()
                 for cmd in values[name]:
                     if cmd[0] in (1, 4):
                         line_ids.add(cmd[1])
                     elif cmd[0] == 6:
                         line_ids.update(cmd[2])
-                # build corresponding new lines, and prefetch fields
-                new_lines = self[name].browse(NewId(id_) for id_ in line_ids)
-                for subname in subnames:
-                    new_lines.mapped(subname)
+                # prefetch stored fields on lines
+                lines = self[name].browse(line_ids)
+                fnames = [subname
+                          for subname in subnames
+                          if lines._fields[subname].base_field.store]
+                lines._read(fnames)
+                # copy the cache of lines to their corresponding new records;
+                # this avoids computing computed stored fields on new_lines
+                new_lines = lines.browse(map(NewId, line_ids))
+                cache = self.env.cache
+                for fname in fnames:
+                    field = lines._fields[fname]
+                    cache.update(new_lines, field, [
+                        field.convert_to_cache(value, new_line, validate=False)
+                        for value, new_line in zip(cache.get_values(lines, field), new_lines)
+                    ])
 
         # Isolate changed values, to handle inconsistent data sent from the
         # client side: when a form view contains two one2many fields that