DIY_GIT_in_Python/how_to/Change_11.md

• write-tree: Write tree objects

Now comes the fun part, where we turn a collection of separate files into a single object that represents a directory.

The idea is that we will create one additional object that collects all the data necessary to store a complete directory. For example, if we have a directory with two files:

$ ls
cats.txt    dogs.txt

And we want to save the directory, we will first put the individual files into the object database:

$ ugit hash-object cats.txt
91a7b14a584645c7b995100223e65f8a5a33b707
$ ugit hash-object dogs.txt
fa958e0dd2203e9ad56853a3f51e5945dad317a4

Then we will create a “tree” object that has the content of:

91a7b14a584645c7b995100223e65f8a5a33b707 cats.txt
fa958e0dd2203e9ad56853a3f51e5945dad317a4 dogs.txt

And we will put this tree object into the object database as well. Then the OID of the tree object will actually represent the entire directory! Why? Because we can first retrieve the tree object by its OID, then see all the files it contains (their names and OIDs) and then read all the OIDs of the files to get their actual content.

What if our directory contains other directories? We’ll just create tree objects for them as well and we’ll allow one tree object to point to another:

$ ls
cats.txt    dogs.txt    other/
$ ls other/
shoes.jpg

The root tree object will look like this:

blob 91a7b14a584645c7b995100223e65f8a5a33b707 cats.txt
blob fa958e0dd2203e9ad56853a3f51e5945dad317a4 dogs.txt
tree 53891a3c27b17e0f8fd96c058f968d19e340428d other

Note that we added a type to each entry so that we know if it’s a file or a directory. The tree that represents the “other” directory (OID 53891a3c27b17e0f8fd96c058f968d19e340428d) looks like:

blob 0aa186b09fd81e8cf449ba10eee6aff9711cc1ac shoes.jpg

We can think about this structure as a tree you know from Computer Science where each entries’ OID as a pointer to either another tree or to a file (leaf node).

Note that we actually save the tree objects with type “tree” in data.hash_object() since we don’t want the trees to be confused with regular files.