Introduction

This project aims to make it easy to see differences in different versions (e.g. git diff) of .xcdatamodel files used to describe Core Data managed object models. An example git diff output follows:

diff --git a/Recipes.xcdatamodel/elements b/Recipes.xcdatamodel/elements
index 35a20f3..939bc61 100644
--- a/Recipes.xcdatamodel/elements
+++ b/Recipes.xcdatamodel/elements
@@ -2,8 +2,8 @@ Entity: Image (NSManagedObject)
   Att: image                     Transformable                                            O   dafQGIhBFATAxE1hyQd5z5cpRg0OLP4+M+W3+xhA6jU=
   Rel: recipe                    Recipe          image                            Nullify   I fVH2lmmkHE4j/FvzfJ2et3KsNxcA8p5BJp2d/xd4hH0=

-Entity: Ingredient (Ingredient)                                                               5tCLlA1r9p1UWdOTr3XJ+ZzBr6bxraWAs3Vt9zCAZek=
-  Att: amount                    String                                                   O   b9/jjR2iJtm4oldVJwj25X+/hpEL6/1CM5hLhgV48Iw=
+Entity: Ingredient (Ingredient)                                                               7nbTTDaWG1lAddHYHum4xP4IA/N90NYv9GLkGK+sueQ=
+  Att: amount                    Integer16                                                O   RmH9Sk61kxsb8+GUEqlEkIuV4tDjxxhMNsHUu/tBW6I=
   Att: displayOrder              Integer16                                                    kMPJ+qU+fnBipO5Ajep+KT3rKB9zeOPrav0q4rMSt7k=
   Att: name                      String                                                       jLmWXAAxrGiROYTzEQlBrZZTlb6f2bF9575UvHrUaJA=
   Rel: recipe                    Recipe          ingredients                      Nullify O I GvmjTsOh76OGkr0Lmnxdh8u6FO4E+iuEYa0mRZPuKJQ=

The project contains a script, print_xcdatamodel, which prints a textual representation of a .xcdatamodel file. This script as the textconv program for a diff driver in git (see Installation below).

print_xcdatamodel works by compiling the xcdatamodel file to a mom file using the momc compiler. The compiled mom file is then read by another program, MOMPrinter, which also ships with this project. MOMPrinter takes a .mom file as input and prints a detailed description of the data model on standard output.

Installation

1. Copy print_xcdatamodel to your $PATH

2. Build MOMPrinter and copy the binary to your path

• Open the MOMPrinter project in Xcode and build with release configuration

• Right click on Products > MOMPrinter in the project navigator, then click 'Show in Finder' to locate the binary

• Copy the binary to your $PATH

  Alternatively, you can build the project on the command line using xcodebuild.

Adding the git-diff driver

1. Ensure that momc is available in your $PATH. For Xcode 4.3.2, the momc binary can be found at /Applications/Xcode.app/Contents/Developer/usr/bin/momc

2. Set up git diff to use print_xcdatamodel for xcdatamodel files

• Add the following line to your projects .gitattributes file (if you want this to apply globally, you can use ~/.gitattributes)

  elements diff=xcdatamodel
  

  This tells to use xcdatamodel as the diff driver for .xcdatamodel/elements files (which contain your model's entire definition). What exactly is this xcdatamodel driver? That is defined in the next section.

• We can set up the xcdatamodel driver using git config. Use these commands to set up the driver (use the --global option if you want to set up the driver for all your projects)

  git config diff.xcdatamodel.xcfuncname ^Entity
  git config diff.xcdatamodel.textconv print_xcdatamodel
  

Now, whenever you use any git command which shows a diff output (e.g. git diff, git log -p, git show, etc.) and there's a change in an xcdatamodel file, you should be able to understand easily what changes have been made to the data model.