ものだと

supposed to

→ 物
phrase Common #6,895
supposed to Used with 思う or a similar expression to describe what someone believes, assumes, or expects to be the case. Often translates as “thought that...” depending on context.
JA: 彼はもう帰ったものだと思っていた。
EN: I thought he had already gone home.

彼は女性の喫煙をいいものだと認めない

He doesn't approve of women smoking.

彼らは笑いながらお互いにつまらぬ誤解をしていたものだと語り合った

They laughed as they told each other about the silly misunderstandings they had had.

その文書彼女の兄さんものだと私たちは考えた

We thought that document belonged to her older brother.

ジョン新鮮な空気良いものだと強く信じている

John firmly believes that fresh air is good for you.

僕たちは彼が死んだものだとあきらめた

We gave up hope and assumed he was dead.

車の中血が女性のものだとなぜわかる

How do you know the blood in the car belongs to a woman?

— ミスティック・リバー (2003)

僕らの血筋そういうものだと信じてるのか

Do you really believe that's what our bloodline is like?

— ダーク・プレイス (2015)

石油は昔から重要なものだとされていた

Oil has long been considered important.

そのハンドバッグからもらったものだとメアリー主張した

Mary claimed that she had received the handbag from her husband.

そこにあるものはすべて泥棒が入れたものだと弁護士は言う

The lawyer says that everything there was put there by the thief.

— ミスティック・リバー (2003)

彼らは死んだものだとあきらめた

They gave up and assumed he was dead.

彼は事態が正しいものだと証明しようとした

He tried to prove that the situation was the right one.