In 1 Kings 1:11-14, Nathan and Bathsheba formulate a plan to have David make Solomon his successor as king rather than Adonijah, a plan which includes an oath:
Then Nathan asked Bathsheba, Solomon’s mother, "Have you not heard that Adonijah, the son of Haggith, has become king, and our lord David knows nothing about it? Now then, let me advise you how you can save your own life and the life of your son Solomon. Go in to King David and say to him, 'My lord the king, did you not swear to me your servant: "Surely Solomon your son shall be king after me, and he will sit on my throne"? Why then has Adonijah become king?'"
Bathsheba then goes in to see David and reminds him supposedly of this oath he made (verse 17):
She said to him, "My lord, you yourself swore to me your servant by the Lord your God: 'Solomon your son shall be king after me, and he will sit on my throne.'"
On the one hand, Nathan is the prophet of God and one would assume a reliable character. Yet, on the other hand, there doesn't seem to be any record of David making such an oath in the books of Samuel. And obviously the whole thing is presented as an intrigue with Nathan and Bathsheba double-teaming David so-to-speak. But, again, when David finally acts, he seems very clear-headed about the whole situation; not like one who has been duped.
Does the author intend us to understand that this oath is fabricated, such that Nathan and Bathsheba are attempting to trick David? If so, is this making some sort of statement about Solomon's rule? Or, is the author leaving the fact of the oath purposefully ambiguous to the reader? And if so, what is the purpose of doing so? Or is the reader simply meant to take it as face value that David made such an oath?