Weird Way to Bring Xcode 8.3.3 Autocompletion Back

I was so upset at the time my project doesn’t seem to work properly. That was not because the application I made doesn’t return the desired result. In fact, it was working properly. But I got confused quickly after the Xcode autocompletion shows me nothing except default keywords.
At the first time, I assumed that it because of the git (yes, I used version control in the folder) doesn’t recognize some files. Xcode warned me about “missing <file> from working copy” and these files flagged with exclamation and question sign indicating that they were untracked by the Git.
I was going ahead to re-stage those files back to git by git add . which I assumed it will fixing the ‘bug’ I’ve just encountered. But then the problem still remains the same. I did some suggestion proposed by peoples in StackOverflow, removed Derived Data, even restart my Mac. Autocompletion still far from me.
But, it is worth noting that this issue has just happened in one project. The rest of my another project remains at their best state which means the problem is just because something went wrong in this project. I try to compare it to another project but I realized that it just waste my time.
Get an idea? Yes, a while after.
That’s surprised me so much! As it turns out, the problem is lying under the Core Date .xcdatamodeld which I just learn at the surface. I just need to remove the file and now I can see the autocompletion back. I tried to reproduce the error or ‘bug’ but I found no ‘bug’ anymore.