Git - Ignoring a specific modification to a config file

Question

I have a config file in our project that holds a connection string to the database, as well as several application settings, e.g:

...
<setting name="ConnectionString"><value>Server=prodServer;Database=myDataBase;</value></setting>
<setting name="AnotherSetting1"><value>Lorem</value></setting>
<setting name="AnotherSetting2"><value>Ipsum</value></setting>
<setting name="AnotherSetting3"><value>dolor </value></setting>
...

During my development, I always change the ConnectionString value to my local database which causes git to put the file mark the file as "modified".

I read about the option of doing the following:

git update-index --assume-unchanged app.config

However, that would mean that if I do ANY other change to the file (e.g, change AnotherSetting1) then git would ignore that as well.

Is there a way to tell git to ignore a specific change in file, but to mark it as modified if any other changes occur? Or is there another similar solution to this problem?

Please assume that I can't make any changes to architecture of the config file itself - I'd like a local-only solution.

Answer 1

filters are made for stuff like this. In your repo,

cat >.git/info/saved-connection <<EOD
<setting name="ConnectionString"><value>Server=prodServer;Database=myDataBase;</value></setting>
EOD

cat >.git/info/my-connection <<EOD
<setting name="ConnectionString"><value>Server=myprivateserver;Database=myDataBase;</value></setting>
EOD

git config filter.use-my-connection.smudge 'sed -f ".git/info/use-my-connection.smudge"'
git config filter.use-my-connection.clean  'sed -f ".git/info/use-my-connection.clean"'

cat >.git/info/use-my-connection.smudge    <<EOD
/^<setting name="ConnectionString">/ {
     w .git/info/saved-connection
     r .git/info/my-connection
     d
}
EOD

cat >.git/info/use-my-connection.clean     <<EOD
/^<setting name="ConnectionString">/ {
     w .git/info/my-connection
     r .git/info/saved-connection
     d
}
EOD

echo >> .git/info/attributes     path/to/app.config filter=use-my-connection