Cakephp 3.0 Save associated model

Conventions, conventions, conventions

There's clearly a difference in the examples that you've linked, take a closer look at the property names, and if you scroll down a little further, you'll find an explanation specifically for belogsTo associations.

When saving belongsTo associations, the ORM expects a single nested entity at the singular, underscored version of the association name. For example: [...]

Cookbook > Saving Data > Saving BelongsTo Associations

So for belongsTo associations, the property name is by default expected to be lowercased and underscored, ie $aClient->client_preference.

Your foreign key should btw. be singular too in order to match the conventions, ie client_preference_id, even though it's just the property name causing the problem.

See also Cookbook > Associations > BelongsTo Associations (especially the foreignKey and propertyName options)