delete_instance() deletes multiple rows in a table
I added a testcase in deef4531b9fc0c87cbecdec8d059650d6e83936c and it is passing. You may need to upgrade your peewee version. What version are you using currently?
 I'm currently on 2.2.2. I'll test with the upgrade ASAP. Thanks for the assistance ;)

```
>>> import peewee
>>> peewee.__version__
'2.2.2'
```
 Yup! That totally fixed it. Thanks again!

```
Current branches: 
----------------------------------------
master 1
foo 1
test-branch 1
Current branch count:  3
Deleting 1 branch:  foo
actual delete count:  1
Remaining branches: 
----------------------------------------
master 1
test-branch 1
Remaining branch count:  2
```

Hello, I've poured over the documentation for delete_instance(), but I found no reference for this behavior I'm currently seeing.

I have two models. `Project` and `ProjectBranch`.

`Project` is just a CharField.

`ProjectBranch` is as follows:

``` python
class ProjectBranch(DBModel):

    name = CharField()
    project = ForeignKeyField(Project, related_name='branches')

    class Meta:
        primary_key = CompositeKey('name', 'project')
```

However, when I call `delete_instance()` on any `ProjectBranch()` object, it deletes all rows from the table.

Example:

``` python
print "Current branches: "
print "-"*40
i = 0
for branch in md.ProjectBranch.select():
    i += 1
    print branch.name, branch.project.id

print "Current branch count: ", i

project = md.Project.get(id=1)

branch_foo = md.ProjectBranch.get(name='foo', project=project)
print "Deleting 1 branch: ", branch_foo.name
print "actual delete count: ", branch_foo.delete_instance()

print "Remaining branches: "
print "-"*40
i = 0
for branch in md.ProjectBranch.select():
    i += 1
    print branch.name, branch.project.id

print "Remaining branch count: ", i
```

Output:

```
Current branches: 
----------------------------------------
master 1
test-branch 1
foo 1
Current branch count:  3
Deleting 1 branch:  foo
actual delete count:  3
Remaining branches: 
----------------------------------------
Remaining branch count:  0
```
