yasnippet : How to expand snippet inside snippet?

Question

In yasnippet's manual expansion of snippet inside a snippet is mentioned but I wasn't able to achieve this. Can someone tell me what I'm doing wrong ?

I have two snippets :

# -*- mode: snippet -*-
# name: newhist
# key: newhist
# expand-env: ((yas-triggers-in-field 't))
# --
${1:-- }----------------------------------------------------------------------------
${2:_head}$2
$1Objet        : ${4:Objet}$0

and

# -*- mode: snippet -*-
# name: _head
# key: _head
# --
${1:-- }Auteur       : `(and (boundp 'user-ngram) user-ngram)`
$1Date         : `(capitalize (format-time-string "%d/%m/%Y"))`
$1Version      : `(and (boundp 'global-release) global-release)`.00.${2:0001}$0

And I expect something like that after expansion :

-- ----------------------------------------------------------------------------
-- Auteur       : me
-- Date         : 29/09/2016
-- Version      : 513.00.0001
-- Objet        : Objet

But all I get is :

-- ----------------------------------------------------------------------------
_head_head
-- Objet        : Objet

Answer 1

you need to add this to your yasnippet config in .emacs to enable nested triggering of snippets

(setq yas-triggers-in-field t)

Answer 2

An alternative to yas-triggers-in-field is to bind yas-expand to a key (I use C-<tab>), and then use this key to expand when you're already in a snippet.

It has the benefit that you'd only get expansion when you want to, TAB would by default not perform expansion when you're already in a snippet (unlike using yas-triggers-in-field).