Cascade deleting with EF Core

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Cascade delete always works in one direction – from principal entity to dependent entity, i.e. deleting the principal entity deletes the dependent entities. And for one-to- many relationships the one side is always the principal and the many side is the dependent.

Looks like you are confused by the fluent configuration. Note that each relationship consists of two ends. The fluent configuration allows you to start with one of the ends and relate it to the other end, or vice versa, but still you are configuring (defining) a single relationship. So 

Entity<A>().HasOne(a => a.B).WithMany(b => b.As)

is the same as

Entity<B>().HasMany(b => b.As).WithOne(a => a.B);

and they both define one and the same relationship. Which one you choose doesn’t matter, just use single configuration per relationship in order to avoid discrepancies.

With that being said, 

model.Entity<Post>().HasOne(p => p.Blog).WithMany(b => b.Posts)
    .HasForeignKey(p => p.BlogId)
    .OnDelete(DeleteBehavior.Cascade);

and

model.Entity<Blog>().HasMany(b => b.Posts).WithOne(p => p.Blog)
    .HasForeignKey(p => p.BlogId)
    .OnDelete(DeleteBehavior.Cascade);

is one and the same and define single one-to-many relationship from Blog to Post. Since Blog is the one side and Post is the many side, the Blog is the principal entity and the Post is the dependent entity, hence deleting a Blog will delete the related Posts.

Reference:

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