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# How-to: Trees

##### Example 1
###### Condition

We have a set of books associated with certain categories.

 ```1 2 3 4 5 6``` ``` CLASS Category 'Category'; name 'Name' = DATA ISTRING (Category); CLASS Book 'Book'; name 'Name' = DATA ISTRING (Book); category 'Category' = DATA Category (Book); ```

We need to build a form with a tree, where the category is shown above and the product is shown below.

###### Solution

 ```1 2 3 4 5``` ``` FORM books 'Books' TREE cb c = Category, b = Book PROPERTIES name(c), name(b) FILTERS category(b) == c ; ```

##### Example 2
###### Condition

Similar to Example 1, except that each category has a parent in the hierarchy.

 `1` ``` parent 'Parent' = DATA Category (Category); ```

We need to build a form with a tree, where the categories will be displayed as hierarchy.

###### Solution

 ```1 2 3 4``` ``` FORM categories 'Categories' TREE categories c = Category PARENT parent(c) PROPERTIES(c) name ; ```

##### Example 3
###### Condition

Similar to Example 2.

We need to create a form with a category tree, so that the books that belong to the current category and all its subcategories are shown to the right.

###### Solution

 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20``` ``` level 'Level' (Category child, Category parent) = RECURSION 1l IF child IS Category AND parent == child STEP 2l IF parent == parent(\$parent) MATERIALIZED; FORM categoryBooks 'Books by category' TREE categories c = Category PARENT parent(c) PROPERTIES(c) name OBJECTS b = Book PROPERTIES(b) name FILTERS level(category(b), c) ; DESIGN categoryBooks { NEW pane FIRST { fill = 1; type = SPLITH; MOVE BOX(TREE categories); MOVE BOX(b); } } ```

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