UIScrollView does not scroll with AutoLayout

I can't make UIScrollView scroll vertically when using Autolayout, I've tried many suggestions. I've read Apple's technical note but even that doesn't seem to work with my pure auto layout approach.

Here is my simple code where I'm adding two UIView blocks1 & 2 in a container view. The container view itself is the only child of the UIScrollView as suggested by many references online. I set the heights of block 1 and 2 as 800 points each, but the scroll view won't scroll.

    - (void)viewDidLoad {
        [super viewDidLoad];


        // Test UIScrollView with Autolayout, scrolling should word

        UIView *mainView = self.view;

        UIScrollView* scrollView = [UIScrollView new];
        scrollView.translatesAutoresizingMaskIntoConstraints = NO;
        scrollView.backgroundColor = [UIColor redColor];
        [self.view addSubview:scrollView];

        UIView* contentView = [UIView new];
        contentView.translatesAutoresizingMaskIntoConstraints = NO;
        contentView.backgroundColor = [UIColor greenColor];
        [scrollView addSubview:contentView];

        UIView* block1 = [[UIView alloc] init];
        block1.translatesAutoresizingMaskIntoConstraints = NO;
        [block1 setBackgroundColor:[UIColor blackColor]];
        [contentView addSubview:block1];

        UIView* block2 = [[UIView alloc] init];
        block2.translatesAutoresizingMaskIntoConstraints = NO;
        [block2 setBackgroundColor:[UIColor blackColor]];
        [contentView addSubview:block2];


        NSDictionary* viewDict = NSDictionaryOfVariableBindings(mainView,scrollView, contentView, block1, block2);

        [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[scrollView]|" options:0 metrics:0 views:viewDict]];
        [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[scrollView]|" options:0 metrics:0 views:viewDict]];


        [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[contentView(==mainView)]|" options:0 metrics:0 views:viewDict]];
        [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[contentView(==mainView)]|" options:0 metrics:0 views:viewDict]];


        [contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[block1(==300)]" options:0 metrics:0 views:viewDict]];
        [contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[block1(==800)]" options:0 metrics:0 views:viewDict]];

        [contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[block2(==300)]" options:0 metrics:0 views:viewDict]];
        [contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-810-[block2(==800)]" options:0 metrics:0 views:viewDict]];

}