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 source compiles to a binary, XCDataModelPrinter, which prints a textual representation of a .xcdatamodel file. This binary acts as the textconv program for a diff driver in git (see Installation below).

XCDataModelPrinter works by compiling the .xcdatamodel file (actually, its a directory) to a .mom file using the momc compiler. The program should usually be able to find momc, but if it can't, you can help it by setting the environment variable MOMC_PATH. Once it has a compiled .mom file, it uses the NSManagedObjectModel interface to print a textual representation on standard output.

Installation

1. Build XCDataModelPrinter and copy the binary to your path

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

• Right click on Products > XCDataModelPrinter 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.

2. Test with an existing .xcdatamodel file.

   XCDataModelPrinter /path/to/your/project.xcdatamodeld/project 1.xcdatamodel

   If you see a textual representation of the data model printed on stdout, skip to the next section. If you see an error that says "Couldn't find momc", you need to help XCDataModelPrinter find the the momc binary. You can do this by setting the environment variable MOMC_PATH to the path of the momc binary.

Adding the git-diff driver

1. Add the following line to your project's .gitattributes file (if you want this to apply globally, you can use ~/.gitattributes)

   elements diff=xcdatamodel

This tells git 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.

2. 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.xfuncname ^Entity.*$ git config diff.xcdatamodel.textconv XCDataModelPrinter

   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.