allow customization of through model relation names

docs/relations/many-to-many.md

@@ -161,7 +161,72 @@ The default naming convention is:
    it would be `PostCategory`
 *  for table name it similar but with underscore in between and s in the end of class 
    lowercase name, in example above would be `posts_categorys`
-   
+ 
+### Customizing Through relation names
+
+By default `Through` model relation names default to related model name in lowercase.
+
+So in example like this:
+```python
+... # course declaration ommited
+class Student(ormar.Model):
+    class Meta:
+        database = database
+        metadata = metadata
+
+    id: int = ormar.Integer(primary_key=True)
+    name: str = ormar.String(max_length=100)
+    courses = ormar.ManyToMany(Course)
+
+# will produce default Through model like follows (example simplified)
+class StudentCourse(ormar.Model):
+    class Meta:
+        database = database
+        metadata = metadata
+        tablename = "students_courses"
+
+    id: int = ormar.Integer(primary_key=True)
+    student = ormar.ForeignKey(Student) # default name
+    course = ormar.ForeignKey(Course)  # default name
+```
+
+To customize the names of fields/relation in Through model now you can use new parameters to `ManyToMany`:
+
+* `through_relation_name` - name of the field leading to the model in which `ManyToMany` is declared
+* `through_reverse_relation_name` - name of the field leading to the model to which `ManyToMany` leads to
+
+Example:
+
+```python
+... # course declaration ommited
+class Student(ormar.Model):
+    class Meta:
+        database = database
+        metadata = metadata
+
+    id: int = ormar.Integer(primary_key=True)
+    name: str = ormar.String(max_length=100)
+    courses = ormar.ManyToMany(Course,
+                               through_relation_name="student_id",
+                               through_reverse_relation_name="course_id")
+
+# will produce Through model like follows (example simplified)
+class StudentCourse(ormar.Model):
+    class Meta:
+        database = database
+        metadata = metadata
+        tablename = "students_courses"
+
+    id: int = ormar.Integer(primary_key=True)
+    student_id = ormar.ForeignKey(Student) # set by through_relation_name
+    course_id = ormar.ForeignKey(Course)  # set by through_reverse_relation_name
+```    
+
+!!!note
+    Note that explicitly declaring relations in Through model is forbidden, so even if you
+    provide your own custom Through model you cannot change the names there and you need to use
+    same `through_relation_name` and `through_reverse_relation_name` parameters.
+
 ## Through Fields
 
 The through field is auto added to the reverse side of the relation. 

docs/releases.md

@@ -24,6 +24,60 @@
     in those cases you don't have to split save into two calls (`save()` and `save_related()`)
   * it supports also `ManyToMany` relations
   * it supports also optional `Through` model values for m2m relations
+*  Add possibility to customize `Through` model relation field names.
+  * By default `Through` model relation names default to related model name in lowercase.
+    So in example like this:
+    ```python
+    ... # course declaration ommited
+    class Student(ormar.Model):
+        class Meta:
+            database = database
+            metadata = metadata
+    
+        id: int = ormar.Integer(primary_key=True)
+        name: str = ormar.String(max_length=100)
+        courses = ormar.ManyToMany(Course)
+    
+    # will produce default Through model like follows (example simplified)
+    class StudentCourse(ormar.Model):
+        class Meta:
+            database = database
+            metadata = metadata
+            tablename = "students_courses"
+    
+        id: int = ormar.Integer(primary_key=True)
+        student = ormar.ForeignKey(Student) # default name
+        course = ormar.ForeignKey(Course)  # default name
+    ```
+  * To customize the names of fields/relation in Through model now you can use new parameters to `ManyToMany`:
+    * `through_relation_name` - name of the field leading to the model in which `ManyToMany` is declared
+    * `through_reverse_relation_name` - name of the field leading to the model to which `ManyToMany` leads to
+    
+    Example:
+    ```python
+    ... # course declaration ommited
+    class Student(ormar.Model):
+        class Meta:
+            database = database
+            metadata = metadata
+    
+        id: int = ormar.Integer(primary_key=True)
+        name: str = ormar.String(max_length=100)
+        courses = ormar.ManyToMany(Course,
+                                   through_relation_name="student_id",
+                                   through_reverse_relation_name="course_id")
+    
+    # will produce default Through model like follows (example simplified)
+    class StudentCourse(ormar.Model):
+        class Meta:
+            database = database
+            metadata = metadata
+            tablename = "students_courses"
+    
+        id: int = ormar.Integer(primary_key=True)
+        student_id = ormar.ForeignKey(Student) # set by through_relation_name
+        course_id = ormar.ForeignKey(Course)  # set by through_reverse_relation_name
+    ```  
 
 ## 🐛 Fixes